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    "source_title": "Encyclopaedia Britannica (1926)",
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    "chunk_id": "1926:fur trade:6dcd1c3cf6f7",
    "title": "FUR TRADE",
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    "verified_text": "the most important development in this field in the period 1910-26 has been the rise of fur farming, made possible through the domestication of the silver fox and other fur-bearing animals. this industry had its inception in prince edward island, canada, those chiefly responsible for its successful development there being r. j. oulton, charles dalton, j. s. gordon, robert tuplin and b. j. and silas rayner. as the silver fox pelt is the most valuable skin produced in north america, these men were able to accumulate great wealth for themselves as well as opening the door to similar opportunities for others. domesticated silver foxes sold at first for as high as $35,000 a pair. prices have since fallen, with the stabilisation of the industry, to less than $1,000. development of the industry—the governments of canada and of the united states have taken cognisance of the new industry by the establishment of experimental ranches and information bureaux. yearly exhibitions also have been instituted for the comparison and judging of the animals. the industry has since spread widely, ranches having been established in great britain, norway, sweden, germany, france and other countries. the domestication of the silver fox has encouraged similar experiments with other animals and these have led to the successful rearing of minks, red foxes, blue foxes, raccoons, karakul sheep (persian lambs), martens and fishers. muskrats and beavers have also been raised in confinement for the sake of their pelts and require a much larger territory in which natural conditions are maintained and enemies excluded. under such conditions these animals multiply very rapidly. markets and production—prior to the world war london and leipzig were the two largest fur markets in the world, more than half the total supply being sold at these places. most of the dressing and dyeing was done in germany. it is estimated that in 1910 north america and europe each produced furs valued 131 at $24,000,000, asia $26,000,000, australia $6,000,000, south america $2,000,000, and africa and oceania $2,000,000. the annual production has, since that time, more than doubled in value, canada producing the greatest amount and exporting in 1925 pelts valued at $17,119,981 (about $22,000,000 in 1919-20). this increase is to be accounted for in large part by the rise in value of furs, occasioned in the first instance by the scarcity of pelts following the war. muskrat pelts, the staple fur of north america, which had formerly been sold for from 10 to 30 cents apiece, frequently brought a price of $5.00. the interruption in trade caused by the war resulted also in the rise of a number of new fur markets, notably in st. louis, new york and montreal. the combined offerings of muskrat pelts at these three cities in the years 1920-3 amounted on the average to 3,366,516 pelts. the combined offerings of all auctions for these skins held in north america for 1924-5 averaged 4,594,446 pelts per year. manufacture in america.—becfore 1914 approximately 65% of the furs secured in north america were exported abroad for manufacture and sale. the difficulties of export during the war, however, resulted in a large increase in fur manufacturing in america and particularly in the united states, where it is largely concentrated in new york city. the total imports into the united states from canada and other countries in 1924 were valued at $87,705,582. about 20,000 concerns in the united states were handling furs in 1924. of these, new york city alone had 2,000 wholesale manufacturers, who disposed of 80% of the manufactured furs sold in the wholesale trade. the fur- manufacturing establishments in the united states increased from 1,300 in 1914, employing 9,000 people, to 1,700 in 1923, with 19,000 employees. the wages paid increased from $6,000,- 000 in 1914 to $28,000,000 in 1923. the estimated retail value of furs and fur trimmings sold annually in the united states is $ 500,000,000. bibliography.—h. m. chittenden, the american fur trade of the far west, 3 vol. (1902); m. petersen, the fur traders and fur bearing animals (1914); g. c. davidson, the north west company (1918); a. c. laut, the fur trade of america (1921); a. m. ahern, fur facts (1922); national association of the fur industry, year book, 1924-5. br. g. hodgson, raising beaver (1924); trapping in northern canada (1925); successful muskrat farming (1925). (r.sge h,*) furtwangler, wilhelm (1886- ), german conductor, was born in berlin jan. 25 1886, the son of the famous archae- ologist and university professor, adolf furtwingler (1853-1907). he received his musical education in munich from beer-walbrun, rheinberger and schillings, and then acted as conductor at ziirich. later he occupied similar positions at strasbourg, li- beck, mannheim, vienna and frankfort-on-main. in1922,onthe death of arthur nikisch, he became director of the berlin phil- harmonic orchestra and of the gewandhaus concerts at leipzig. ranked among the finest conductors in germany, furtwiingler also gained an international reputation, which was considerably enhanced by his annual-concerts in new york. fuse: see ammunition. futurism: see painting. 132 aelicg literature: see irish literature, gaelic; scottish-gaelic literature. } gairdner, james (1828-10912), british historian (see 11.390), died at pinner, middlesex, nov.4 1912. a third volume of his lollardy and the reformation in england appeared in irgil. galatz or galat1, rumania (see 11.396), saw considerable in- dustrial development before the world war. the population, 73,512 in 1914, was estimated in 1919 at 60,000. the export of grain and timber has revived, and there are grain- and flour-mills in the town. the port has been improved, and vesscls of 5,0c0 tons can reach it since dredging on the danube has been resumed; there is now from 204 to 23 ft. at low water. the docks accom- modate ro steamers and 20 barges. the communications by road and rail, however, are unsatisfactory, and are jiable to be interrupted altogether in winter. galatz is the seat of the rumanian naval school. the rumanian naval base is at sulina. (see danube.) gale, zona (1874- }, american author, was born at portage, wis., aug. 26 1874. having graduated from the uni- versity of wisconsin, she engaged in newspaper work in milwau- kee, and in rgor joined the staff of the world (new york). after 1904 she devoted herself to fiction, verse and plays, producing among other works, the loves of pelleas and ettarre (1907); friendship village stories (5 vol., 1908-19); birth (1918); afiss lulu bett (1920); the secret way, verse (1921); and faint per- fume (1923). she was awarded a pulitzer prize for her dramati- sation of miss lulu bett, produced in new york, 1920. galicia, east (see galicia, 11.401), a district assigned to poland in 1923. its oil-fields are of great value. the province of galicia, finally assigned to austria in 1815, fell into two well- defined ethnographical parts. east of przemysl, some 72% of the population was ruthenian (ukrainian), the minority being mainly composed of poles and jews, who are extraordinarily numerous in the towns. it was usually contended by the polcs of galicia that ruthenes do not exist; and the austrian govt. long accepted this theory in order to ensure the loyalty of the poles. the russian govt. also supported this view in order to discourage the nationalist movement in the ukraine; and the ruthenes themselves were divided—one party fecling itself russian, the other ruthenian-ukrainian. about ro910, when the neo-slav movement partly reconciled the poles with the rus- sians, thus shaking their loyalty to austria, austria began to encourage the particularist ruthenian movement as a counter- blast to panslavonic ideas. during the war east galicia was the scene of heavy and con- tinuous fighting. during 1916-7, in view of the importance of poland for the central powers, it was proposed to unite all galicia with russian poland in a kingdom of poland more or less closely attached to austria-hungary. but after the collapse of russia, the central powers concluded a peace with the ukraine at brest-litovsk (feb. 9 1918) and austria-hungary promised to erect east galicia, with the bukovina, into a separate austrian territory (kronland). the ruthenes claim independcnce—while the claims of the poles to autonomy were universally recognised, those of the ruthenes were forgotten. in oct. 1918 the austrian govt. hav- ing promised to introduce a federal system, the polish leaders claimed for themsclves all galicia. the ruthenes replied that they would rather “ fight and die than let themselves be annexed by poland.” they summoned a conference of representatives of all ruthenes of austria-hungary to meet at lemberg (lvov, lwow, leopol) on oct. 18. the socialist party pressed for imme- diate union with the ukraine; but the bourgeois parties formed a national council which determined to form an independent state, inviting the polish and jewish minorities to co-operate. on nov. 1: the council occupied the government buildings in lemberg with the help of ruthenian regiments, and assumed the gaelic literature—-galicia, east government of east galicia within its ethnographical limits. the poles refused their co-operation, and fighting began between poles and ruthenes. the austrian govt. recognised the ruthe- nian govt. which was now formed under dr. eugen petrushiewicz, and handed him over a share of the old imperial assets; but the poles immediately invaded east galicia, occupying lemberg on nov. 5. the ruthenian govt. retired to vienna, and its activi- tics now became very platonic, being chiefly concerned with propaganda, in which it developed prodigious activity. meanwhile on jan. 3 1919 the hetman petlura, who at that time had established his authority in the ukraine, proclaimed the union of russian ukraine and west ukraine (fast galicia) in a republic. the ruthenian troops joined the ukrainian army with the consent of the east galician govt., which recognised petlura’s authority on jan. 20. fighting continued throughout 191g, during which time polish troops occupied east galicia, in de- fiance of the efforts of the supreme council to arrange a truce and s-ttle the problem. on may 8 the supreme council assigned west galicia to poland; but on june 24 it decided to give east galicia the right of self-determination, although authorising poland to proceed with her military occupation of the district. the. east galician govt. revoked its recognition of ukrainian authority on aug. 28 1919, and continued to assert the legality of its own position and the right of east galicia to self-determination. in dec. 1919 the allied and associated powers announced that east galicia would be granted autonomy for 25 years under a polish protectorate, after which time the league of nations would decide on its future. this remarkable arrangement evoked vehement protests from the ruthenes. it also caused a crisis in poland, which desired unconditional possession of the district, and resulted in the fall of paderewski. petrushiewicz’s govt. was, however, powerless, and the numerous ukrainian governments which existed simultaneously or consecutively were fully occupicd in war with each other, the poles, the bolsheviks or the white russians. in april 1920 petlura abandoned his claim to east galicia in return for recognition by poland; petlura’s and the polish armies captured kiev from the bolsheviks on may 8; but by nov. the bolsheviks had driven petlura out of the ukraine and established themselves there firmly. in galicia, however, the poles held their own, and the treaties of riga, oct. 12 1920 and march 18 1921, re-established the old frontier of galicia with the ukraine. petrushiewicz’s govt. was in a disastrous position. it was ignored by everyone; its funds were running out; its choice of allies lay between its hereditary racial enemies, the poles, and bolshevik ukraine. the poles meanwhile treated the district as their own; held elections to the polish diet in it in the autumn of 1922—although practically the whole ukrainian population abstained from voting—and even pressed ruthenes into the polish army. on march 14 1923 the conference of ambassadors recognised poland’s de facto eastern, northern and southeastern fronticr. the pelish regime.—political activity in east galicia did not lose its customary complexity after this date. many polit- ical leaders, including petrushiewicz, came to terms with the soviet authorilies. the great bulk of these unfortunates were unable to return to their homes. a small minority accepted the situation, and attempted to combine with the other racial minor- ities in poland. a general law passed by the polish diet on sept. 26 1922, which established local government bodies to deal with purely local affairs, contained the proviso that in the east galician provinces these bodies should consist of two chambers, of which one should be ruthenian, this “ autonomy,” as it was described, was to be introduced within two years; the language rights of minorities in poland were to be respected, and coloni- sation forbidden. but in fact, although a language law was passed on july 10 1924, providing that ukrainian, white rus- sian or lithuanian should be the language of administration, galli-curci—gambia law-courts and schools in districts where the majority of the population speaks one of these languages, and count skrzynski showed a not illiberal attitude towards the minorities generally, polish policy in general was not hampered by an excess of scruples towards the ruthenes. (c. a. m.) galli-curci, amelita (188 9— ), italo-american singer, was born at milan, italy, nov. 18 1889. as a vocalist she was self-taught. gifted with a singularly pure soprano voice, she made her debut as gilda in rigoletio at the constanzi theatre, rome, in 1909, and subsequently visited spain and south amer- ica. she made her first appearance in the united states at the auditorium, chicago, nov. 18 1916, where she remained four seasons. she was engaged by the metropolitan opera company, new york, in 1921 and from that date constantly appeared in grand opera and concerts. among her chief reles were butter- fly, juliette, lucia, mimi, rosine, violetta and manon lescaut. she made a concert tour through great britain in 1924-5. gallieni, joseph simon (1849-1016), french general and statesman (see 11.418), reached the age limit for retirement on april 24 1914, but was retained on the active list without duty, and on july 31 1914 was officially confirmed as the eventual suc- cessor of the commander-in-chief. on aug. 26 he was appointed governor of paris and general commanding the armies of paris. he took energetic steps for the defence of the capital, and on sept. 3 issued the following order: “ to the army of paris and the population of paris: the members of the government of the republic have left paris in order to give a new impulse to national defence. i have been ordered to defend paris against the invader. this order i shall fulfil to the end.” after sept. 2 the armed forces in paris had been placed under the orders of joffre, who could thus in case of need use the gar- rison for his operations; consequently, on sept. 3 gallieni wrote to joffre asking for instructions. from information received during the day, however, gallieni concluded that the german forces marching on paris were inclined southeast, thus offering him an exposed flank. joffre’s answer to gallieni’s messages arrived on the 4th a little before 3 p.m. and contemplated an attack south of the marne. gallieni held that to gain the fullest possible advantage from the situation by enveloping von kluck and breaking his lines of communication the attack ought to be made on the line of the ourcq north of the marne, and made at once. gallieni spoke to joffre on the telephone on the 4th at 5 p.m. and obtained authority to move on the sth north of the marne for the attack on the 6th. the vi. army under maunoury was placed under gallieni’s command. maunoury was given all the troops available ‘and ordered to move on the 5th to the heights of the ourcq, to attack on the 6th. but, while executing this movement, maunoury was himself attacked on the 5th by the german iv. reserve corps, which covered von kluck. the two armies were almost equal; six divisions against six. but during the night of the 7th-&th gallieni sent up the 7th division in taxis from paris to nanteuil-le-haudouin. the germans re- treated and the french armies moved out of the zone over which gallieni held command. on oct. 29 1915 gallieni became minister of war in briand’s cabinet. his period of office was made notable by endeavours to create unity of command and by changes in the position of gen. joffre. he resigned for reasons of ill-health on march 16 t916 and died at versailles, after undergoing two operations, on may 27. his body was given a state funeral, and lay for a time under the dome of the invalides. he was buried at st. raphael. on april 21 1921 the dignity of marshal of france was conferred on him posthumously. gallipoli: see dardanelles campaign. galsworthy, john (1867- ), british playwright and novelist, was born at coombe, surrey, aug. 14 1867. educated at harrow and new college, oxford, he was called to the bar in 1890, but devoted himself to literature. his first novel, joce- lyn, appeared in 1898, but he attracted wider attention in 1904 with the island pharisees, and the man of property (1906). the latter was the first novel of the sequence to be known later as 133 the forsyte saga, the others being the indian summer of a for- syte (1918); in chancery (1920); awakening (1920); and to let (1921). asa detailed picture of upper middle-class society dur- ing the later victorian and edwardian eras, the sequence is a remarkable achievement. that picture was supplemented by the greater number of galsworthy’s other novels, of which it is the characteristic social setting, among them the country house (1907); fraternity (1909); the patrician (1911); the freelands (ro1s); the white monkey (1924). galsworthy also wrate the dark flower, 1913, and many short stories, of varying merit, but often worthy of his best work, collected as caravan (1925), essays on current social and moral questions, marked by liberal and humane feeling, and a commentary on the extravagances of war time, the burning spear. as a dramatist also, galsworthy enjoyed a deserved fame. his plays, for the most part, are based on ethical or social prob- lems and are marked by a scrupulously judicial fairness in dis- playing the opposing points of view typified by his characters. some have partaken too much of this scrupulousness to be counted as having an essentially dramatic quality, but at their best galsworthy’s plays reflect not only his finest powers as a literary craftsman—for their diction is natural and serious with- out becoming loose or facile—but also a keen sense of dramatic values. among them should be noted the silver box (1906); joy (1907); strife (1909); justice (1910); the pigeon (1912); the eldest son (1912); the fugitive (1913); the skin game (1920); loyalties (1922); the forest (1924). see s. kaye-smith, john galsworthy (1916); andre chevrillon, trois etudes anglaises (1924). | galton, sir francis (1822-1911), british anthropologist (see 11.427), died at haslemere jan. 17 rg1t. galvanauskas, ernest (1882-— ), lithuanian states- man, was born nov. 7 1882 at vabalninkai. he studied as a mining engineer at leningrad (st. petersburg) and liege, and his activities in the lithuanian national movement date from his student days. in 1906 he was imprisoned for his attempts to free the country from the russian yoke. he escaped to belgium in 1908, where he continued in touch with the lithuanian libera- tion movement. the outbreak of the world war found him in belgrade, and after the enemy occupation of that city, galva- nauskas left with the serbian army and shared all the privations of the retreat. he subsequently proceeded to france and vol- unteered for military service. on the recovery of lithuanian independence in r9r18, galvanauskas served as a member of the lithuanian delegation to the versailles peace conference. he was several times prime minister and minister for foreign affairs, and also held the portfolios of finance and communica- tions. he became president of the lithuanian delegation to the league of nations and in aug. 1924 was appointed lithu- ania’s first envoy extraordinary and minister plenipotentiary to london and simultaneously envoy extraordinary and minister plenipotentiary to the hague. in nov. 1925, for services to the serbian cause in the world war, he received from the king of yugoslavia, the high military order ‘‘ albarrakom spozeincom.”’ gambia (see 11.4374), a british colony and protectorate in west africa. its population was 210,530 in 1921, an increase of 60,000 in io years. bathurst, the capital, had 9,227 inhabitants, of whom europeans numbered 265 (130 british). hemmed in landward by french territories, which make but little use of its magnificent waterway, gambia depends upon its own resources. the cultivation of the ground nut, entirely a native industry, is the chief occupation of the people. up to rors the nuts (the oil from which is a main ingredient of margarine) went mostly to france, which in to14 took 78% of the crop. changes caused by the world war led to over o1 % of the export going to england in 191g. yrance subsequently in part recovered the market. in 1924 england took 54° and france 33% of the crop, which represented some 95 % of the total exports. the economic crisis caused by the war was safely weathered and 1920 was a year of trade activity. imports are mostly cotton piece goods, rice and sugar and, from sierra leone, kola nuts. great britain’s share of the imports averaged over 50% in the years 1914-24; france, ger- 134 many (since 1920) and the united states supply most of the other imports. in 1922 it became necessary to demonctise the french five france piece, the favourite coin of the natives, and this involved the administration in a loss of £187,000. taking 1t914 and 1924 as normal years the revenue rose from £86,000 to £208,000, the expenditure from £120,000 to £203,000. the tonnage of shipping entered and cleared was 1,100,000 in 1924 compared with 571,000 in r9r4. british shipping pre- dominated, but from tg21 germany became a keen competitor and in 1924 was second, with 158,000 tons as against 643,000 british, 93,000 american and 92,000 french. since 1901 gam- bia has enjoyed political tranquillity, and education and chris- tianity made progress, if slowly. most of the natives are mos- lems. the gambia co. of the west african frontier force served with credit in the campaigns in the cameroons and ger- man east africa. bathurst was the first place visited by the prince of wales in his 1925 tour. bibliography.—h. f. reeve, the gambia (1912), an excellent monograph by a retired official; sir g. denton, ‘‘ twenty-three years in lagos and the gambia,” jour, african soctety, vol. 11 (1912); the gambia (1920), a british forcign office handbook, and the annual report's issued by the colonial office, london. (f. r. c.) game preserves: see national parks. gandhi, mohandas karamchand (1869- ), hindu nationalist leader, was born at porbandar (kathiawar, india) of a bania family with official traditions. at the age of 19 he went to london, studied for a time at university college, and was called to the bar by the inner temple. soon after starting practice in the bombay high court he was called, in 1893, on professional business, to south africa, where he threw himself at once into a long and bitter struggle for the liberties of the indian settlers in that country. he became leader of the opposi- tion to a variety of measures taken by the local authorities to discourage asiatic immigration and to limit the rights of citizen- ship available to asiatics already resident. it was in the course of this movement that his conception of resistance without violence developed; he relinquished his large income as an advocate, and founded a colony for his com- patriots on tolstoian lines near durban. as the price he paid for his championship of the indians’ grievances, besides being more than once arrested and imprisoned he suffered frequent indignities at the hands of angry mobs. this neither checked his energies nor deterred him from rendering service of marked loyalty to the government on three occasions; for he raised and commanded a red cross unit in the boer war, he organ- ised a plague hospital when the epidemic broke out in johannes- burg, and he led a stretcher-bearer party in the suppression of the natal revolt of r908. at last in r914 a commission of in- quiry into the indian discontent recommended the removal of several of the worst injustices against which gandhi had striven; and he felt justified in closing down his activities in south africa and returning to india. there a wider field of political protest awaited him; and he was soon at work organising, in connection with the home-rule movement, resistance to the british government by ‘ soul force’ and nonco-operation. in jan. 1919 there were published two bills ( subsequently known as the rowlatt acts) giving the government emergency powers for dealing with revolutionary crimes and conspiracies; these had followed the proposals of a responsible commission which had investigated the subject, and the powers they conferred were safeguarded by elaborate protections against abuse. but gandhi declared them to be an insult, intended to discredit the indian people on the eve of ostensible political reforms, and he denounced the bills as instruments of oppression. he instituted a campaign of satyagraha (literally, insistance on truth) or non- violent disobedience to unjust laws in the first instance, en- larging if necessary into disobedience to any law and complete nonco-operation with the government. spreading rapidly, the agitation burst into violence in the punjab and elsewhere, with results which shocked gandhi into a temporary suspension of his civil disobedience. later in the year, he formed common cause with the indian moslems of the khildfat party, aggrieved game preserves—garden by the terms of peace which great britain was offering to turkey; and in july 1920 he proclaimed a general campaign of *“non-violent nonco-operation.” its points were the boy- cott of government service, of the new legislatures and of the courts of law; the surrender of all public offices; and the with- drawal of children from government schools; to which were subsequently added boycott of foreign goods and the adoption of the spinning-wheel as an emblem of economic independence. the agitation spread rapidly. the unlettered people who saw his earnestness and asceticism, and heard his simple elo- quence, regarded him as a saint, and invested him with the title of mahatma, or great soul. by 1921 gandhi was at the zenith of his power. the national congress, sitting at christmas of that year, delegated its full authority to him, and empowered him to appoint his own successor. but signs of change were now appearing. ‘the unrest into which the non-co-operation move- ment had plunged the country culminated in a series of grave outrages, some of a racial character, of which the moplah outbreak was the worst, and others directed against the agencies of law and order. gandhi met them by personal penances, and by repeated postponements of the date on which he had fore- told that india would be liberated from british rule. but he had generated forces which he had no power to recall or control, and plain people were getting alarmed at the consequences. moslem support was being deflected from him by the encour- agement which lord reading’s government was giving to islamic sentiment over turkey; and his closest adherents were embarrassed by his frequent changes of policy. consequently, when he was arrested in march 1922 and put on trial for con- splring to spread disaffection with a view to overthrowing the government of the country, the coup evoked little excitement. gandhi pleaded guilty, accepted responsibility for all that had happened and invited “ the highest penalty that can be in- flicted upon me for what in law is a deliberate crime and what appears to me to be the highest duty of a citizen. ”’ condemned to six years’ simple imprisonment, he was re- leased in jan 1924 after an operation in gaol for appendicitis, and the rest of his sentence was unconditionally remitted. he came back to a party which had chosen other leaders, and was soon to reverse his policy of boycotting the administration. esteemed and consulted though he continued to be by his old followers, he was no longer a power to sway the masses, and at the end of 1925 he announced his intentions of retiring from the world for a year. the nationalist movement was switched on to lines more familiar to western constitutional usage. to the oriental mind gandhi symbolised self-sacrifice and high ideal- ism; western enthusiasts were not wanting who, like m. romain rolland, compared him with the founder of christianity; and the transparency of his character commanded the personal liking of even strong opponents. but he expected too much of human nature, he plaved recklessly with inflammable material, his economic nostrums were unpractical and he gave way to unexpected bursts of intolerance, as when he described the british government of india as “‘ satanic.”’ his endeavour to unite hindus and moslems lacked sincerity; but his enduring legacy to indian politics will be the part he has taken in con- solidating hindu nationalism. see romain rolland, mahatma gandhi (paris, 1924); eng. trans. by c. d. groth, 1924. (me. garden, mary (1877- ), american operatic singer, was born at aberdeen, scotland, feb. 20 1877. at the age of six she was brought to the united states. in 1888 her family settled in chicago, where her early musical training was received. she went to paris in 1896, and studied under trabadello, chevallier and fougere. she made her debut at the opera comique, panis, april 3 1900 in the title rele of louise, taking the place, at a few days’ notice, of mlle. riotou, who had fallen ill. her first appear- ance in the united states was at new york in the title rele of thais nov. 25 1907. in 1910 she became a member of the chi- cago grand opera company, assuming, among others, the parts of salome, thais, melisande and louise. she was appointed general director of the chicago opera association jan 13 1921. garden city—gary, ind. garden city: see city planning. garibaldi, giuseppe (1879- ), italian general, eldest son of gen. ricciotti garibaldi and grandson of the liberator, was born at melbourne july 29 1879. he fought under his father in the greco-turkish war in 1897, and served in the south african war, subsequently leading an adventurous life in south and central america and in the balkans. on the out- break of the world war he raised an italian legton of 14,000 men, which fought on the side of france in the argonne. in 1915 garibaldi returned to italy, and on italy’s entry into the war he enlisted as a volunteer. tle was soon afterwards com- missioned, being given command of a battalion, and served with distinction inthe lv. army. he returned to the french front in march 1918 in command of the famous brigata alpi, and in june was promoted brigadier-general. he resigned his command in june 1919 and gave up his commission in feb. 1920. he be- came opposed to the fascist govt. and in the autumn of 1924 was involved in an anti-fascist agitation organised by the alia libera association. deciding, however, that the movement had no chance of success, he departed for new york to devote himself to business. garland, hamlin (1860- ), american writer, was born at west salem, wis., sept 16 1860. he graduated from the cedar valley seminary, osage, ia., at the age of 21. after teach- ing in illinois and taking up a claim in south dakota (then dakota territory), he went to boston in 1884 and engaged in literary work. the drudgery of farm life in the middle west and the adventurous life of the mountains furnished him with abun- dant material for his realistic tales. his works, all based on the life of the northwest states, include main travelled roads (1890- 8); ulysses s. grant: his life and character (1898); boy life on the prairie (1907); the long trail (1907); amfoney magic (1907); cavanagh (1009); other main travelled roads (1913); the tyran- ny of the dark (1905); a son of the middle border (1917); a daughter of the middle border (1921); the book of the american indian (1923). he was made a member of the american academy of arts and letters tn 1918. garstin, sir william edmund (1849-1925), british en- gineer, was born in india jan. 29 1849. he was educated at cheltenham and king’s college, london, and in 1872 entered the indian public works department. in 1885 he was transferred to egypt and in 1892 became inspector-general of irrigation and under-secretary of state for public works. he proved himself an indefatigable worker, and by his efforts extensive areas were reclaimed in egypt and the sudan. among his great- est works were the asyut barrage and the aswan reservoir. in addition, it was due to him that the white nile was cleared of sudd, thus rendering possible free navigation between khartoum and gondokoro. he wascreated k.c.m.g. in 1897 and g.c.m.g., in 1902. in 1907 he was appointed british govt. director of the suez canal company. during the world war he devoted him- self to red cross work in england, being created g.b.e. in 1918. he died in london jan. 8 1925. garvin, james louis (1368- ), british publicist, and editor-in-chief of the present volumes of the lncyclopedia britannica, was born at birkenhead, ches., april 12 1868. a political writer from the age of 17, he joined the newcastle chronicle, where from 1891 to 1899 he was leader-writer, literary critic and aide-de-camp to its proprietor, the celebrated orator, joseph cowen. from the age of 26 for many years he was a chief contributor to the fortnightly review, and later to the national review and the ouarterly. in 1899 he went to london, joining the staff of the daily telegraph, which gave him unusual scope. when mr. joseph chamberlain started his historic cam- paign for tariff reform and imperial unity, mr. garvin became nationally known as that statesman’s most powerful heutenant in the press. editor in 1905-6 of the weekly review the outlook, he passed in 1g08 to the editorial chair of the observer. he raised that oldest of great sunday journals to the highest posi- tion it had ever reached, and made it quoted throughout the world. simultaneously from 1o12 to 1915 his editorship gave new life to the evening pall mail gazette. steeped in german 135 studies, his articles on foreign affairs, anticipating the world war from as early as rgor, had a deep public influence. in the war his articles were famous; but, equally the advocate of “absolute victory ’’ and a wise peace, he was a searching and damaging critic of the treaty of versailles. to promote inter- national reconciliation and co-operation he wrote in six weeks during the paris conference (1919) his elaborate book tie economic foundations of peace. his only son, gerard, who had justified high hopes, was killed at the age of 20 in the battle of the somme, gallantly leading two companies of the south lan- cashire regiment in a night action. behind all mr. garvin’s politics there is a deep background of history and literature. in 1926 he was chosen chairman of the empire press union. gary, elbert henry (1846- \\, american jurist and busi- ness man, was born near wheaton, il., oct. 8 1846. he attended wheaton college and then, after studying law for a time in an office, he continued his legal studies at the university of chicago, and took the degree of ll.b. in 1867. in 1871 he began practice in chicago, where he became a noted corporation lawyer. in 1874 he organised the gary-wheaton bank, of which he was president. he was elected judge of du page co. in 1882 and again in 1886; was three times elected president of the town of wheaton, and on its becoming a city in 1892 served as mayor for two terms. he was president of the chicago bar association 1893-4. he early saw the advantages of combination in business and in 1891 was one of the organisers of the consolidated steel & wire co. in 1898, upon the organisation of the federal steel corporation, with a capital stock of $200,000,000, he became its head and retired from legal practice. this company was merged in the united states steel corporation in 1901 with a capital stock excecding $1,000,000,000 and he was elected chairman of the board of directors and of the finance committee. he con- tinues to fill these offices after twenty-five years of remarkable de- velopment of the steel industry, and growth of the corporation. the town of gary, ind., laid out in 1906 as a model home for steel workmen, was named in his honour. in 1914 he was made chair- man of the committee appointed by mr. mitchel, mayor of new york, to study the question of unemployment and its relief. when america entered the world war in 1917, gary was appointed chairman of the committee on stecl of the council of national defence. through his own connection with a business essential for munitions of war he exerted great influence in bring- ing about co-operation between the government and industry. ile was interested in a strengthening the friendship between america and japan. in 1919 he was invited by president wilson to attend the industrial conference in washington, and took a prominent part in it as a firm upholder of the “open shop,” of which he was always a strong advocate. he also opposed the drastic restriction of immigration effected by the ‘ quota ” law. miss ida m. tarbell’s life of judge gary concludes with these words: ‘‘ he has made a lasting contribution to our difficult and often baffling problem of substituting, in american business, bal- ance for instability, mutual interest for militarism, co-oper- ation for defiance, frankness for secrecy, goodwill for distrust. no man in contemporary affairs has more honestly earned the high title of industrial statesman.” gary, ind., u.s.a. (see 11.480), had become in 1920 a city with 55,378 inhabitants of whom 5,299 were negroes and 16,- 460 foreign-born. in 1925, the 20th year of its existence, its population was 81,888 (school census); it had an area of 40°83 sq. m., 155:35 m. of improved and 178-45 m. of unimproved streets, 15 m. of boulevards, 45 m. of street railway track, a fleet of motor-buses and 94 m. of water-mains. the assessed valuation of taxable property was $131,006,810. there were over 1,000 retail stores. during the nation-wide strike of the steel workers in 1919, gary was occupied by federal troops from oct. 7 until the strike was called off on jan. 7 1920. a city- planning commission was appointed in 1919 and a comprehen- sive zoning ordinance was adopted in 1925. the “ gary gate- way and railway developement,” which was adopted in 1924, provided for improved freight and passenger facilities and for beautifying a large section. 136 the public schools of gary offer academic instruction 48 weeks in the year and physical training for 52 weeks of 63 days each week. the first superintendent of schools, william a. witt, worked out a system with the following features: (1) an en- riched curriculum; (2) a school plant adapted to such curriculum, with playground, gymnasium, swimming-pool, shops, labora- tories and auditorium; and (3) organisation of classes on a “‘ platoon ” system, by which each feature of the plant is in use all the time, thus providing for the most economical utilisation of both plant and teaching force. this administrative scheme (3) became known among educators as the ‘‘ gary plan” and was adopted, with more or less modification, in many cities. see f. p. bachman and r. bowman, the gary public schools, a study by the gencral education board in eight parts (new york, 1918). gas in warfare: sce chemical warfare. gas, manufacture of (see 11.483)—in the period 1910-25 the work done on gas retorts, coke ovens, gas generators, the complete gasification of coal, low temperature carbonisation and other operations in the gas industry has been considerable. during the war, gas-retort and coke-oven plants were worked to their full capacity; repairs and renewals were often unavoid- ably deferred or effected under difficult conditions. gas and coke manufacturers have studied nearly every branch of the in- dustry, and therefore many important changes have taken place. therms.—in great britain the provisions of the gas regulation act of 1920 impose an obligation on the suppliers to charge for gas at a price based on the calorific value of the gas and specified the use of a recording calorimeter for determining this value, the calorific unit sclected, the therm, is equal to 100,000 b.t.u.’s or british thermal units. each gas undertaking formally declares the calorific value of the gas it proposes to supply; this is called its ‘' declared calorific value '’ and is usually from 450 to 500 b.t.u.'s per cubic foot of gas. the method of reading the gas meter remains the same, but the gas account is made out in therms at so much per therm. the number of therms is easily computed, for, suppose that the meter readings show that 10,800 cu. ft. of gas have been supplied and that 500 is the declared calorific value, then 10,800 x 500+ 100,000= 54 therms. at tenpence per therm, the gas bill would be £2°5-0. . the act allows a gas undertaking to declare any calorific value it chooses. therefore, subject to the requirements of gas purification, the company is free to adopt gas-making processes which will yield the maximum number of therms for every ton of coal treated. clearly, it may be most profitable to supply a mixed gas, or to leave in the gas constituents which, like benzol, have a high thermal value; with gas and benzol at their present prices, benzol is usually left in the gas. calorimeters.—among the recording calorimeters used by gas undertakings are the simmance total heat, the fairweather, and the boys, in all of which the heat of combustion of known quantities of gas to be tested is imparted to a flow of water; the thomas, in which the heat is imparted to air; and the beas- ley, in which a differential thermometer filled with oil is em- ployed. at large gasworks, several calorimeters are used, one in connection with the town’s gas supply and the others with the separate gas-making plants. refractory matcrials—in great britain, siliceous clays con- taining 80 to 92% of silica and silica clays with more than 92% of silica have been used to an increasing extent in the gas indus- try and carborundum has also been tried. for the hotter parts of vertical retorts, siliceous and silica clays are commonly used and silica clays have been employed largely in constructing the coke-oven plant of the consett iron co., ltd., and other com- panies. in america the use of silica clays is the standard prac- tice, not only for the hotter parts of vertical retorts and for the chamber ovens of the koppers and other types for supplying gas, but also for horizontal retorts. _ silica bricks are more fragile, and have a coefficient of expan- sion greater than that of firebricks. -their greater heat conduc- tivity facilitates rapid carbonisation; they sustain high tempera- tures and keep their shape in a satisfactory way. it is probable gas in warfare that the life of silica is greater than that of fireclay retorts and ovens. at st. helens, eight glover-west retorts, in which friden silica was used, had an average working life of 1,694 days. some of the american chamber ovens, constructed by messrs. kop- pers, have worked continuously for 15 years and are said not to have needed repairs (see electro-metallurgy). carborundum is very refractory, hard and strong; its coeffi- cient of expansion is very small; its heat conductivity is about five times that of fireclay. the future of this valuable refractory material will depend largely on its initial cost, and on its behav- iour in an oxidising atmosphere at high temperatures. william- son, cliff, ltd., of stamford, have constructed a horizontal retort with a tile of carborundum extending along the central part of the retort sole. in the united states, carborundum has been used in both retort and gas generator construction. the retort controversy —the question of the best type of gas retort has been decided to a large extent by the selections made by numerous gas engineers when erecting new installations of retorts. there are in- stallations of inclined retorts working satis- factorily and numerous installations of hori- zontal retorts provided with efficient charging and discharging machines, but the tendency is clearly in the direction of using continuously operated vertical retorts. s a ee adil re an, hu hlorvizontal retorts—the improvements as made since 1910 in these retorts and their bz, settings have been mainly in details. some \\y in continuously operated horizontal retorts se» 1 ’ z==% erected by west’s gas improvement co., ltd., _/=(e y van shor as rennany ssaa g ass iz oanaaaaa — reer ttt a yore nn nanralsneulrny at blackpool and middleton, did not show any advantages over the glover-west vertical retort, and were abandoned. vertical retoris——the chief improvements oy in vertical retorts have been in connection with those of the continuously operated type, in which coal is fed into the top of the retort and coke discharged from the bottom in a con- tinuous and regular manner, the retort in- creasing in area downward to facilitate descent of the charge. the glover-west and the woodall-duckham vertical retorts are the chief examples. the glover-west continuous vertical retort was originally developed on principles deduced from the practice with scotch shale retorts. in its most approved form, the 4o-in. new wy... model, the cross-section is a flattened ellipse, \\s bie the major axis of the mean section being 40 zi inches. vertically, the retort enlarges regularly until the steaming chamber or “ bell-bottom,” clearly shown in fig. 1, is reached. ‘this chamber opens up the core of coke and permits the ex- posure of a large surface of coke to the action of steam injected for making water gas. a powerful helix mounted on a vertical shaft, shown in fig. 1, supports the full weight of the coke; it is driven by worm gearing and, as it revolves, the coke is lowered continuously er-west vertical and regularly into the coke chamber, with a retort. minimum of breakage. a step-grate producer generates gas which is led to combustion chambers arranged horizontally across the lines of retorts. the temperature distribution can be readily varied over the whole length, about 25 ft., of the retort, according to the carbonisation requirements of the coal treated. he glover-west was successfully worked at the st. helens gasworks in 1909. since then, more than 135 gas under- takings in all parts of the world have adopted it; the new model has been installed recently at oldbury, nine elms (gas light and coke co.), dudley leicester, toronto and malden, united states of america. in the woodall-ducxham continuous vertical retort the z zz 7 , anskasaaatteanan ne 5 z yz any z 4 nasa sass x z aaranueaaranaq sssonss 4 z ss y 4 sanaaaaaas z 4 va saw > z qs rn mp rss wf, jl an of saree se fic. 1.-—-glov- gas, manufacture of continuous and regular discharge of the coke is effected by an extractor roller formed of a series of stars set on a horizontal shaft slightly in advance of each other, so as to form helical blades. the shaft is mounted in a gas-tight hopper, having one side curved so as not to prevent descent of the coke by gravita- tion while supporting the greater part of its weight. heating of the retort is effected by means of a regencrative gas producer. there are two very hot combustion chambers, one at the top of the retort and another lower down for ensuring that the charge is hot enough for obtaining good water gas by the action of steam admitted to the retort. the woodall- duckham retorts were installed at the bournemouth gasworks, 1903-5; since then, numerous installations have been set up in all parts of the world. among the most recent are those at dun- fermline, glasgow, edinburgh, warrington and bergen. coke ovens.—the course of development in coke-oven practice has been chiefly in the utilisation of surplus gas for town supply, and in the installation of very large ovens of known types. in great britain parliamentary sanction was first given in 1910 for the supply of coke-oven gas. the board of trade returns for 1924 gave 16 towns taking gas from colliery and other coke- oven plants, chesterfield, sheffield, leeds, middlesborough and pontypridd being included. most coke ovens in great britain do not take a charge exceeding 8 tons, but ovens of the piette type, of nearly 18 tons capacity, have been built at stoke-on- trent by the semet-solvay and piette co. of sheffield. in the united states, it is common practice to use coke ovens of about 15 tons capacity. charging and discharging.—the introduction during recent years of efficient charging and discharging apparatus has done much to enable horizontal retorts to compete with other types of retort. many charging machines, e.g., the arrol-foulis hydraulic charger and the de brouwer, which projects the coal into the rctort, are worked in association with a separate discharger. drake's projector and pusher is a combination of a pusher made up of large, jointed sections, adapted to fold over a rotary, polygonal drum and a rapidly rotating wheel with vanes for projecting coal into the retort; the pusher discharges the retort and is withdrawn, and then the pro- jector comes into operation. a further development is a type of apparatus called a discharging charger, of which guest and gibbons’ discharging charger is an example; it is described in british patent specification no. 103143/1916. a sectional, jointed trough which, as it travels forward from its supporting wheel, discharges the coke, has mounted within it rollers supporting and guiding an endless drag- bar chain. during the withdrawal of the trough, this chain is driven se heat and carries coal into the retort so as to fill it com- pletely. steaming practice.—steam has been used, especially in the dessau intermittent retorts, to increase the yield of ammonia. during recent years, however, the practice has been greatly ex- tended, especially in working continuous vertical retorts, which are admirably adapted for steaming. the methocl admits of numerous easy variations; usually superheated steam under a pressure of 10 to 40 lb, per sq. in. through a 4-in. nozzle. as long as the steaming is not excessive, it increases the yield of gas, ammonia (g¢.v.) and tar, and also the total thermal yield, although the calorific value per cu. ft. of gas is lowered. coal blending.—the blending of different kinds of coal has long been practised, especially in america, to obtain suitable charges for use in coke ovens. coal blending has recently attracted attention, especially in connection with smokcless fucl production. it has been proposed to mix several kinds of coal so as to obtain a blend having a predetermined resinic content or yielding a predetermined amount of volatile matter when heated. by this blending, it is sought to minimise the expansion of the coal when carbonised at low tem- perature, to facilitate discharge by preventing the coke from sticking to the retort, and to obtain a coke of good quality. processes of this kind are described in british patent specifications nos. 164104 and 186085 (illingworth) and 187336 (roberts). coal cleaning.—the removal of clay, shale, pyrites and other impurities from coal by washing processes is important, especially in preparing charges for ovens used in making metallurgical coke, and in connection with smokeless fuel production to facilitate the making of a good quality fuel. the catalytic action of some of the inorganic constituents during carbonising processes, is also receiving attention. carburetted water gas.—during recent years, increasing quan- tities of water gas, especially carburetted water gas, have been made in great britain. in america, more than one-half of the gas manufactured is carburetted water gas; philadelphia is sup- plied almost entirely with this gas. 37 in the self-clinkering, self-steaming generator of humphreys and glasgow, ltd., an annular water jacket forms the lower part of the generator and supports an upper firebrick part; the jacket is high enough to prevent adhesion of clinkers to the generator wall. a steam reservoir communicates with the jacket by pipes, and serves as a source of steam supply for the generator, a mechanically operated grate, having an upper conical part, formed with air passages, and a lower cylindrical part, is mounted eccentrically on the base of a rotary water pan. a central conduit conducts the air blast or steam or carries away the down-run gas, when the run is downward, for the run may be upward or downward as desired. during itsrotation, the grate crushes and expels the clinker and ash, the carbon content of which has been reduced as much as possible. the speed of the grate is regulated automatically; this regulation may be effected by electric means, the action of which is controlled by the temperature beneath the grate. when carburetted water gas is to be produced, the gas is led from the gencrator to a carburettor supplied with a spray of heavy oil and thence to a super-heater in which the cracked oil-vapour is fixed. many of these generators are working in association with batteries of horizontal or vertical retorts, the whole representing a very advanced development of gas-making plant. the coke ch | tee a: a iw {}—1 lll elie till ai elis ill ili 4 fic. 2.-—self-steaming blue water-gas plant. (see text.) from the retorts, in a cold, clean state, is charged into the pro- ducers and the operation of the whole installation effects a sub- stantially complete gasification of the coal. the loss of heat of the incandescent coke is of comparatively small importance, for the clean, cold coke is rapidly brought to an incandescent state in the generator and there is no expensive handling of hot coke. a self-steaming, self-contained, blue water-gas plant, erected by humphreys and glasgow, ltd., comprises (a) a generator, (b) a recuperator, (c) a waste-heat boiler and (d) a washer- scrubber (sce fig. 2). this firm has brought into commercial use a means for automatically operating a generator plant, in which a series of valves governing the interdependent operations neces- sary in working, e.g., coke feed, air supply, steam supply, dura- tion of blow, etc., is operated by hydraulic power in an order correct in time and sequence. the order of operation, deter- mined by the conditions of temperature, pressure, etc., in, or connected with, the generator, is set once for all by adjusting the indicators of a series of dials corresponding respectively with the various valves. if any valve fails to respond in its turn, the cycle of operations is stopped automatically by the action of electric circuits; this prevents explosions or other accidents. complete gasification of coal—complete gasification of coal in two steps is commercially successful. numerous attempts have been made to effect complete gasification in one operation, but none appears to have been successful commercially. one of the earliest attempts was made many years ago by fahnehjelm in the u.s.a.; his apparatus was a compact structure com- prising a vertical conical retort, surrounded by heating flues and supported by a generator below. one of the many difficulties of this apparently simple process was that of obtaining in the 138 generator a regular supply from the retort of incandescent coke suitable for yielding gas. a great deal has been achieved in ger- many by extensive trials of strache’s apparatus, which is gener- ally similar to fahnehjelm’s. the problem is, in fact, receiving careful attention and complete success may be achieved. low temperature carbonisation (see fuel problems).— this is a process for the partial destructive distillation of coal at temperatures which are low compared with those of an ordi- nary gas retort or coke oven; an important aim of the process is to obtain a smokeless fuel suitable for burning in ordinary grates. about 1865, experiments on partial destructive distilla- tion of coal were made by mr. scott-moncrieff, but the process which has led to a more intensive study of the subject is that patented by mr. thomas parker (british specification no. 14,365 of 1906), for subjecting bituminous coal to destructive distillation at a temperature of about 800° f., until illuminating gases cease to be evolved, then stopping the distillation and quenching the charge. this process, usually carried out in tubu- lar iron retorts, was tried repeatedly and many improvements were made, especially by low temperature carbonising, ltd., whose retort plant, set up near barnsley, operates at a tempera- ture between 600° c. and 800° c. e other investigators have worked processes of this kind. the south metropolitan gas co. (london) has made experiments on low temperature carbonisation with a view to obtain a good quality smokeless fuel. an installation of rotary, inclined re- torts, developed by mr. nielsen, has also been worked near barnsley, the temperature of working being about 675° c. by these processes, a smokeless fuel, burning freely with a clear flame, and almost odourless, has been obtained, together with a moderately large quantity of a rich gas, and good yields of tar and ammonia. no plant, however, has been working, on a com- mercial scale, long enough to justify an assertion that the process is a success. any plant for which it is claimed that the problem of low temperature carbonisation has been solved may be tested, free of cost, by the fuel research board, so that he possibilities of the process are still being considered. bergius process—a process which has sttiacied a great deal of attention, especially in germany, is the bergius process, elab- orated by prof. bergius. experiments have been made at mannheim and recently efforts have been made to carry out the process commercially. the chief aim of the process is to obtain oil from coal and it is claimed that coal of poor quality can be used advantageously. the coal is pulverised, mixed with oil residues from a previous operation, and heated to a temperature of about 450° c. in an atmosphere of hydrogen under a pressure of about 150 atmospheres. the product is a viscous mass from which the oils can be separated. gas purification —during the 15 years under review numerous improvements in the details of construction of gas purifying apparatus have been made, especially in connection with the grids and valves of purifier boxes. in modern gasworks water- cooled tubular condensers, bubble washers of the livesey type, the pelouze and audouin type of tar extractors, and rotary brush scrubbers of the holmes type are largely in use. attempts tointroduce new purifying processes have been made. jn birkheiser’s process, two rather small oxide purifiers, working together, are used, the crude gas being passed through one for removal of sh: while a blast of air is passed through the other to revivify the oxide by converting its absorbed she into so.2; this so, is used to recover ammonia. this process and a later modification were tried at the tegel gasworks, berlin, and also at bournemouth, but, apparently, not with complete success. processes have been introduced by w. feld, all ingenious, but most of them too complicated for commercial use. in one, used with success in germany, the reagents employed are a solu- tion of feso, for washing the crude gas and, in later steps of the cycle of operations, a mixture of soz and air. both sh: and nhz3 are eliminated from the gas, the ultimate products being ammo- nium sulphate and sulphur, which is filtered off, and thus recov- ered in solid form. gases, electrical properties of gasholders.—some exceptionally large gasholders have been erected during recent years. the largest gasholders in the world appear to be those of the astoria gasworks, new york. two gasholders in course of erection at birmingham are intended to have five lifts and a total capacity of 20,000,000 cubic feet. a water- less, or tankless, gasholder recently erected in michigan, is a chamber of polygonal cross section with a capacity of 1,000,000 cubic feet. the chamberis closed by a piston, which moves upand down within it. a tar seal forms a gas-tight joint between the chamber and its piston and is a very important part of the whole construction. in countries where the winters are severe, a water- less gasholder offers advantages. (t.e.l.) gases, electrical properties of (see 6. 864). these vary very greatly with the conditions to which the gas is exposed. a gas in its normal condition is practically a non-conductor of electricity even though it be the vapour of a good conductor like mercury. on the other hand, when it is exposed to such in- fluences as cathode rays, rentgen rays, intense electrical forces or the radiation from radioactive substances, it becomes a con- ductor of electricity. the very delicate methods now at our dis- posal reveal however a trace of conductivity in gases where pre- cautions have been taken to shield off the effects just mentioned. the most important electrical property of a gas in a normal state is its specific inductive capacity. the significance of this property is best illustrated from the relation (k—1)/47r =nm, between k, the specific inductive capacity; n, the number of molecules per unit volume, and m, the electrostatic moment which a molecule acquires under unit electric force. as we know n, we can if we know the value of a deduce the value of m, and this will tell us a good deal about the shape and size of the molecule. for example, if we regard the molecules as solid conducting spheres, m=r* where r is the radius of the sphere. thus, on this hypothesis we can find the radius of the molecule if we know the value of a, and though the hypothesis itself does not throw much light on the structure of the atom, it is probable that the radius of a conducting sphere which would produce the same electrical moment would be of the same order of magnitude as the linear dimensions of the molecule: the radii of metallic spheres which would give the specific inductive capacities possessed by hydrogen, nitrogen, oxygen and chlorine, are respectively i-19 x10-8, 1-60x 1078, 1-48 x 10-8, 2-04 x 10-8 centimetres. on the more probable hypothesis that the atoms and molecules consist of electrons arranged round centres of positive electricity, the electric force will displace the electrons relatively to the positive centres and thus cause the molecule to have a finite electrical moment. the more rigidly the electrons are connected to the positive charge, the smaller will be this moment and the smaller the specific inductive capacity of the gas. the values of a—1 for the elements belonging to the same family are connected by a remarkably simple and interesting re- lation, which was discovered by cuthbertson ( phil. trans. a.207, p.135). it is shown in table i. where the numbers under the sym- bols denoting the elements are the values of 3 (ak —1) x 10°:— ‘table: t. he 144x3 n o f ne 297 270 192 137 5 cl ar 1197 iio! 768 568 =299 x4 =275x%4 = 192 x4 =142x4 as se br. kr 1550 1565 1125 850 = 258 x6 =261 x6 =187x6 =142x6 te [ x 2495 1920 1378 =249 x10 =192xi0 =138xi0 thus the values of k—1 for successive elements of the same family—(n, p, as): (o, s, se, te): (ff, cl, br td: (ne, ar, kr, x)—are in all cases very nearly in the proportion 1, 4, 6, 10. in the simple theory, where the molecules are regarded as conduc- tors, this would indicate that the volumes of the molecules of the gases, electrical properties of successive elements in the same family are in the proportion tr, 4, 6, 10, for each of these types of clements. on the theory which regards the atom as built up of electrons arranged round positive centres, the configuration of the outer layer of electrons for dif- ferent members of the same family would be similar, and it is easy to show that for similar configurations of electrons the value of a—1 would be proportional to the cube of the linear dimensions, 7.e., to the volume enclosed by the outer layer of _ electrons; so that again on this theory cuthbertson’s result shows that volumes of successive elements in the same family are in the same ratio whether the family be that of the inert gases, the halogens, or the oxygen or nitrogen groups. w. l. bragg (phil. mag., 40, p.169) using the method of rentgen ray crystal analysis has measured directly the diameters of the atoms of many of the elements. he finds that the cubes of the diameters of f, cl, br, and i are proportional to 1, 3:72, 5-45 and 8 respectively; those of o, s, se to 1, 3-9 and 6, and those of c and si to 1, 3-6. thus for these electronegative ele- ments the ratio of the volumes of corresponding elements in two different periods is approximately constant. this, however, is not true for electropositive elements, where the sizes of the atoms in one period increase much more slowly from one element to the next than they do for the electronegative elements. another example of the information as to the nature of the molecule afforded by determinations of the specific inductive capacity is that, while the specific inductive capacity of many gases, ¢.g., hz, nx, qo, co, cos, ch, is equal (as maxwell’s electromagnetic theory of light suggests) to the square of the refractive index, there are, as badeker (zeitschrift physik. chem., 36, p.305) has shown, others, such as nh3, hci, sos, and the vapours of water and the alcohols, whose specific inductive capacity is far in excess of the value given by this rule, and more- over the specific inductive capacity of these gases diminishes much more rapidly as the temperature increases than that of gases of the first type. the difference can be accounted for by supposing that the molecules of gases of the first type have no electrical moment when they are free from the action of an external electrical force, while those of the second type have an intrinsic electrical moment apart from that which may be pro- duced by the external force. when there is no electrical ficld, the collisions between the molecules will cause the axes of elec- trical moments of the different molecules to be uniformly dis- tributed, so that the average effect will be zero. an electric force will tend to drag the axes of the different molecules into alignment, and the assemblage of molecules will have a finite electrical moment which will be a measure of the specific induc- tive capacity. inasmuch as the collisions between the molecules tend to knock their axes out of line and diminish the specific inductive capacity, the latter will diminish as the temperature and with it the vigour of the encounters increases. the sub- stances which have an intrinsic electrical moment have excep- tionally active chemical properties and are good solvents, dissociating the salts dissolved in them. if the distribution of electrons in a molecule were not sym- metrical about three axes at right angles to each other, the specific inductive capacity of a single molecule would vary with the direction of the electric force, but as the molecules in a gas are orientated in equal numbers in all directions we should not detect this by direct measurements of the specific inductive capacity. we can however detect this effect in another way; for if the molecules have different specific inductive capacities in different directions the light scattered by the molecules at right angles to the incident unpolarised light will not be plane polarised as it would be if the molecule were symmetrical (j. j. thomson, phil. mag., 40, p. 393), and if the incident light is plane polarised the scattered light will not vanish in any direction. strutt (now lord rayleigh) (proc. roy. soc., 98a, 57) has measured the departure from plane polarisation for different gases with the results shown in table ii. this shows that the molecule of argon is very symmetrical, while the nitrogen molecule is more symmetrical than that of oxygen, and this again more symmetrical than that of cos. table ii. argon . 0-46 % hydrogen 3°83 % nitrogen 4:06 % air 5 00 . re) oxygen an ad 9-40 % carbon dioxide . 1z-70% nitrous oxide 15-40 % tontsed gases.—gases may in various ways be put into a state in which they conduct electricity on an altogether different scale from the normal gas. they acquire this conductivity when rentgen rays or the rays from radioactive substances pass through them, or when they are traversed by cathode or positive rays. ultra-violet light of very short wave length can impart this property to a gas, while gases recently driven from flames or from near arcs or sparks or bubbled threugh certain liquids or passed slowly over phosphorus also possess this property. the conductivity of gases possesses interesting characteristics. in the first place it persists for some time after the agent which made the gas a conductor has ceased to act; it always however di- minishes after the agent is removed, in some cases very rapidly, and finally disappears. the conducting gas loses its conductivity if it is sucked through glass-wool, or made to bubble through water. the conductivity may also be removed by making the gas traverse a strong electric field so that a current of electricity passes through it. the removal of the conductivity by filtering the gas through glass-wool or water shows that the conductivity is due to something mixed with the gas which can be removed by filtration, while the removal of the conductivity by the electrical field shows that this something is charged with electricity and moves under the action of the electric force. since the gas when in the conducting state shows as a whole no charge of electricity the charges mixed with the gas must be both positive and nega- tive. we conclude that the conductivity of the gas is due to the presence of electrified particles; some of these particles are positively, others negatively, electrified. these electrified par- ticles are called ions, and the process by which they are produced ionisation. the passage of electricity through a conducting gas does not follow the same laws as the flow through metals and liquid scale divisiong & 0, 100 200. 300.400 500.600700800.9001000. 1160. 12001300. its vo fig.1 electrolytes; in these the current is proportional to the elec- tromotive force, while for gases the relation is represented by a graph like fig. 1, where the ordinates are proportional to the current and the abscissae to the electromotive forces. we see that when the electromotive force is small, the current is propor- tional to the electromotive force, as in the case of metallic con- duction; as the electromotive force increases, the current after a time does not increase nearly so rapidly, and a stage is reached where the current remains constant in spite ef the increase in the electromotive force. there is a further stage, which we shall consider later, where the current again increases with the elec- tromolive force, and docs so much more rapidly than at any previous stage. the current in the stage when it does not de- pend upon the electromotive force is said to be saturated. the reason for this saturation is that the passage of a current of elec- tricity through the gas involves the removal of a number of ions proportional to the quantity of electricity passing through the gas. thus the gas is losing ions at a rate propertional to the cur- rent; it cannot go on losing more ions than are produced, so that the current cannot increase beyond a critical value which is proportional to the rate of production of ions. this sometimes 140 produces a state of things which seems anomalous to those ac- customed to look at conduction of electricity exclusively from the point of ohm’s law. for example, when gases are exposed to rentgen rays, the number of ions produced per second is proportional to the volume of the gas, so that, if two parallel plates are immersed in such a gas and a current sent from one to the other, when the distance between the plates is increased the number of ions available for carrying the current and there- fore the saturation current will be increased also. thus the appar- ent ‘‘ resistance ”’ will diminish as the length of the gaseous con- ductor is increased. the nature of the ions.—the question arises, what is the nature of the particles which carry the charges of electricity? are they the atoms or molecules of the gas, or, for the negative charges, electrons? information on these points is alforded by measuring the velocity with which the ions move through the gas under given electric forces. the ion under the action of the electric force will soon attain a steady velocity which will be determined by the condition that the momentum communicated to the ion by the electric force in unit time is equal to the momentum lost by the ion in the same time by its collisions with the molecules of gas through which it is moving. suppose that the average momentum lost by the ion per collision is gm where m is the mass of the ion, z its velocity and g a quantity which will depend on the relative masses of the ion and the molecule and also on the force between the ion and the molecules, e.g., if the ions and molecules behaved like elastice spheres g would not be the same as if they acted upon each other with forces varying inversely as the fifth power of the distance between them. ‘the value of qg will also depend on the electric charge on the ion. the number of collisions made by an ion with the molecules of the gas in time 6 may be written as 6//t. where t is the average interval between two collisions, 7 will vary inversely as the number of molecules in unit volume, 7.¢., inversely as the density of the gas and will also depend upon the temperature and the masses of the ions and molecules. when the loss of momentum by collisions balances the gain from the electric force f gm not kel = —- i (x) fer or 4 = -—— ng we can determine by experiment the value of #/f and hence the value of t/iq. this will give us information as to the nature of the ions, e.g., we can by the kinetic theory of gases calculate the values of t and g when the ion is a molecule of the gas and when the collision between an ion and a molecule is the same as that between two molecules; on these suppositions the calculated mobility is for most gases considerably greater than that found by experiment. the difference is to be explained by the effect of the electric charge on the collision and also the clustering of other molecules round the charged one to form a more complex ion. these two effects are not independent for it can be shown that if the effect of the charge is large enough scriously to affect the mobility, it follows by the laws of thermodynamics that the forces between the ions and the molecules must be great enough to lead to considerable aggregation at room temperatures. the effects to be expected from the formation of such aggregates may be illustrated from the expression for 2, the mobility of the ion, 7.e., the velocity of the ion under unit force, deduced on the assumption that the force between the ion and the molecule varies inversely as the fifth power of the distance between them, this expression is e (mitm:)3 ——————=«_ $° “2b - antsipans von abe where m,, mz are the masses of the molecule and ion respectively, n the number of molecules per unit volume, yz the refractive in- dex of the gas, s the sum of the radii of an ion and a molecule and 66 the energy of a molecule due to thermal agitation when @ is the absolute temperature of the gas. for most gases the effect of = gases, electrical properties of the term s?v 286 is insignificant. when this is so the effect of the complexity of the ion is represented by the term m ae mm, this varies but slowly with the complexity, for when the ion con- sists of a single molecule m.=m; and the expression is vv 2/m, while when the ion consists of an inhnite number of molecules the valueis ¥1/m, thus this great increase in mass only reduces the mobility in the proportion of 1 tov2. the formula given above for & may be used to find the value of m./m, and for positive ions in most cases this ratio is found to be greater than one. this indicates the formation of complex ions; these under the impact of the molecules break up and new aggregates are formed around the simple ions. this constant change of partners by the ions explains the very remarkable fact that in a mixture of different gases all the ions appear to move with the same velocity. thus in a mixture of a light gas like hydrogen with a heavy one like methyl iodide we do not find ions of great mobility cor- responding to the hydrogen and others of smaller mobility cor- responding to the methyl iodide. wellisch has shown that all the ions in the mixture move with the same velocity. if there had been stable ions with masses comparable with the hydrogen molecules and others comparable with the methyl iodide molecule they could easily have been differentiated. if however the ions form aggregates and the partners of the ions in these aggregates are constantly changing so that during the life of the ion it passes backwards and forwards through a great number of different phases, then though at any particular instant one ion might have a different mobility from another, yet the average mobility taken over a finite time would be the same for all the ions. to detect the different kinds of ions it would be necessary to observe not their average behaviour over a long time but to make the measurements so quickly that the ion had not time to pass from one state to another whilst it was under observation. the life of an ion in a particular phaseis, atatmosphceric pressure, probably considerably less than the millionth of a second; the length of life will vary inversely as the pressure so that the chance of detecting the different kinds of ions would be much greater at low pressures than at high ones. lately erikson has detected a case when the velocity of the ion changes even though the gas through which it is moving remains the same; he has shown that the mobility of the positive ions in alr at atmospheric pressure is, in the first second or so of their existence, about 1-89 but as the ions get older the mobility falls to 1-35, the ratio of these numbers is nearly equal to ¥2 which is the ratio of the mobility of an ion consisting of one molecule to that of one consisting of a very large number. it would thus seem that the old ions are much more complex than the young ones. the surprising thing about erikson’s result is that he found the ions to retain their youthful properties for the best part of a second at atmospheric pressure; during this time the ion will have collided with many million molecules of air, and so might have been expected to form partners in a very small fraction of a second. the results are consistent with the idea that the ion forms a stable connection with the molecules of some very heavy gas present in very small quantities as an impurity. the addition to the gas of small quantities of the vapours of water or alcohol whose molecules have a finite electrical moment produces a marked diminution in the mobility, indicating that they give rise to aggregates of exceptional permanence. formula (2) is obtained on the assumption that the force between the ion and the molecule is due to electrostatic induction alone and so varies inversely as the fifth power of the distance; if the gas were one like water vapour or ammonia whose molecules are polar, the force between the ion and the molecule would vary inversely as the fourth instead of the fifth power of the distance and the mobility would not be given by formula (2). we see from this formula that unless m./m, depends upon the pressure the mobility ts inversely proportional to n, i.e., to the pressure of the gus. this law has been verified for positive ions gases, electrical properties of down to pressures less than 1 mm. of mercury. as however the probability of forming complex aggregates is greater at high than at low pressures, we should expect m2/m, to diminish as the pressure diminishes; this diminution in m2/m, would produce a small increase in the mobility so that the formula would indicate that the mobility would increase just a little more quickly than the reciprocal of the pressure. the behaviour of negative ions is in many respects quite different from that of the positive ones. in the first place the mobility of the negative lons is for the permanent gases greater than that of the positive; thus, for example, in dry hydrogen the velocities of the negative and positive ions, when the electric force is one volt per cm., are 7-95 and 6-7 respectively, and for air 1-87 and 1-36. the difference is less for moist gases than for dry, while for complex vapours which have comparatively small mobilities wellisch found that there was very little difference between the mobilities of the positive and negative ions. for the permanent gases the ratio of the mobilities of the negative and positive ions varies but little with the pressure, un- til the pressure is reduced below that represented by about ro cm. of mercury. for lower pressures than this, the mobility of the negative ion increases, as langevin showed, more rapidly than that of the positive; at the pressure of a millimetre orso the mobility of the negative ion in air may be three or four times that of the positive. an even more interesting result was discovered by franck and hertz, who, when they experimented with very carefully purified nitrogen or argon, found that the mobility of the negative ion was more than roo times that of the positive. the mobilities in these gases are extremely sensitive to traces of oxy- gen, and a fraction of 1% of oxygen added to the pure gas will reduce the mobility of the negative ion to less than one-tenth of its maximum value. the enormous mobility of the negative ion in nitrogen and argon as compared with that of the positive shows that in them the negative electricity must be carried by electrons and not by atoms or molecules, while the eflect of introducing traces of oxygen shows that these electrons readily attach themselves to the molecules of oxygen though they are unable to adhere to molecules of nitrogen or argon. the same effect has also been observed in helium and hydrogen. these properties of the negative ion are of great importance in connection with the mechanism of ionisation in gases and the structure of atoms and molecules. in the first place, they furnish strong evidence in support of the view that the first stage in the ionisation of a gas is the ejection of 4n electron from the molecule of the gas rather than the separation of the molecule into atoms of which some are charged with positive and others with negative electricity. on this view the negative ion begins its career as an electron and not as an atom, while the positive ion from the beginning is of molecular dimensions. as an elec- tron has much greater mobility than a molecule the mobility of the negative ion will at first be much greater than that of the positive. in some gases, such as oxygen, the electron soon gets attached to a molecule, and its mass and mobility become com- parable with those of the positive ton. the mobility we measure is the average mobility of the negative ion during its life; part of the time its mobility, being that of an electron, is very much larger than that of the positive ion, while in the other part the two mobilities will be much the same. the excess of mobility of the negative over the positive ion will depend upon the fraction of its liie which the negative ion spends as a free electron—a fraction which would tend to increase as the pressure of the gas diminished. the mobility of the electron may be estimated by formula (2); if the gas is nitrogen we must in that formula put m2/mi= 1/28 1800 which is the ratio of the mass of an electron to that of a nitrogen molecule. using this value for m.2/m, we find that the mobility of the electron is about 220 times that of the positive ion, a result which is confirmed by experiment. direct experiments on the mobility of negative ions do not give any indication of the existence of two kinds of these ions. this may be regarded as a confirmation of the explanation of the i4i similar result for the positive ions, viz.: that our methods only give the average mobility of the ion through a comparatively long life; for there can be but little doubt that in all but a very few gases the negative ion can exist in two phases, one of them being the electron. the mobility in the electron phase is so exceptional that anything which prolongs the duration of this phase in com- parison with that of the other phases must have a great effect upon the mobility. some observations by loeb (phys. revicw, 17, p-89) give us some information about the time an electron in oxygen requires to unite with the molecule and form a negative ion. he found that the electron made about 50,000 collisions with an oxygen molecule before combining with it and that this ‘number is independent of the pressure. the time taken to make these collisions, taking the velocity of the electron at o°c. as 107 and the free path as 1to~* at 760 mm., is 5 x1078 sec. and this will measure the life of the free electron. it ts evident that, even allowing for the great mobility of the electron, it could not appreciably affect the mobility in experiments lasting for a time comparable with -or sec., and the mobility would be prac- tically that of the negative ion, unless the electron were able to break away again from a molecule after it had united with it. the mobility of the carriers of negative electricity as compared with that of the positive one is evidence that the electron is continually changing partners. if &1 be the mobility of the electron, ko that of the negative ion, t, the life of the electron, t2 that of the negative ion, the average mobility will be hiti + et? lea if is the pressure of the gas, 41 and ke vary inversely as the pressure, let them equal ki/p, ko/p, then the mobility is equal to net | e since loeb’s experiments show that the electron makes the same number of collisions before uniting with the molecule what- ever be the pressure, and since the interval between each collision is inversely proportional to the pressure, t; varies as 1/p, if the breaking up of the negative ions were due to collisions with the molecules t2 would also vary as 1/p and the ratio of t; to ts would be independent of the pressure. this from equation (3) would make the mobility of the negative ion like that of the positive vary inversely as the pressure and would not explain the abnormal increase of the negative carricrs at low pressures. hence we conclude that the breaking up of the negative ions is not due to collisions. if tz is independent of p, while t; increases as p diminishes the product of the mobility and pressure will by (3), since &, is greater than /:, increase as p diminishes, which is in accordance with the results observed by langevinand others. the difference between the behaviour of the electron in nitrogen or argon and in oxygen discovered by franck and hertz is of great importance in connection with the structure of the atom and molecule, for it indicates that, while a molecule of oxygen can accommodate another electron in addition to those already present, the molecules of nitrogen and argon are unable to do so. it is instructive therefore to consider the results ob- tained by the study of the positive rays as to the power of the atoms and molecules of the different elements to acquire a negative charge. these show that, while the atoms of hydrogen, carbon, oxygen, fluorine or chlorine readily acquire a negative charge, those of helium, nitrogen, neon and argon do not; and again that, while it is very exceptional for a molecule whether of a compound or an clementary gas to acquire a negative charge, the molecule of oxygen is able to do so. we see that this result is in accordance with the behaviour of the carrier of the negative charge in an ionised gas. since the atoms in the positive rays show so much greater affinity for the electrons than the mole- cules, it follows that if the agent producing ionisation were to dissociate some of the molecules of the gas into neutral atoms (and to do this would require the expenditure of much less energy than to ionise the gas), these atoms would be much more eifective traps for the electrons than the undissociated molecules. loeb 142 has shown that even in oxygen an electron collides on the average with about 50,000 molecules of oxygen before it is captured; thus if the oxygen afom could capture an electron at the first en- counter, if only one molecule in 50,000 were dissociated into atoms, the effect of the atoms would be as efficacious as that of the molecules in capturing the electrons. when this dissociation into atoms takes place the abnormal velocity of the negative ion will only occur in gases like nitrogen and the inert gases whose atoms cannot receive an elcctron. recombination of the ions—evcen when the ions are not re- moved from a gas by sending a current of electricity through it, their number will not increase indefinitely with the time of exposure of the gas to the ionising agent. this is due to the recombination which takes place between the positive and negative ions; these ions as they move about in the gas some- times come into collision with each other, and by forming elec- trically neutral systems cease to act as ions. the gas will reach a steady state with regard to ionisation when the number of ions which disappear in one second as the result of the collisions is equal to the number produced in the same time by the ionising agent. if there are » ions of either kind per cu. cm., the number of collisions between the positive and negative ions in one second in a cu. cm. of the gas will be proportional to n?; hence the number of ions of either sign which are lost by recombination in one second may be represented by az? when a is called the coeffi- cient of recombination. if the ionising agent produces q ions per cu. cm. per sec., then dn dt the solution of this equation, if we reckon ¢ from the instant the ionising agent begins to act, so that »=o when t=o, is k?=g/a b( e-kat 1) ; we see that, when the gas reaches a steady state, n=k= — = vv g/a, and that the gas will not approximate to this state until tis large compared with 1/22, 1.e., to 1/2 2a where #9 is the value of 2 in the steady state. thus when the ionisation is very weak it may take a considerable time for the gas to reach a steady state. when the ionising agent is removed, the ions do not disappear at once, but decay at the rate given by the equation dn tie —an’, the solution of this, when ¢ is the time which has elapsed since the removal of the ionising agents, and m9 the number of ions when /=0, is = g—an’, ” = no/(1+noaz). thus the number of ions will be reduced to one-half their initial value after a time 1/a%. we may therefore take 1/an as the measure of the life of an ion when there are 2 ions per cubic centimetre. the values of a/e, where e is the charge on an ion, have been measured by various experimenters; the results are given in the following table: the agent used to ionise the gas is given in the second row. table iii. values of afe for various gases at atmospheric pressure and ordinary temperaiure town-| mc- | lan- i ‘ send |clung| gevin| kill | dren a rumelin gas |——|-——- | —— |__| ———_ | —— | roint- | r6nt- | rent- | rent- . . p b gen | gen { gen | gen ; rays | rays | rays | rays | \"#7\" | \"495 | tays | fays air 3,420 | 3,380 | 3,200 | 3,580 | 3,300 | 4,200 | 4,240 | 5,820 no 3,520 | 3,490 | 3,400 | 3,500]... - as - 2 oh gases, electrical properties of the results ascribed to thirkill were obtained by extrapolation from experiments made at lower pressures. since e, in electro- static measure, is 4-8 10-!9, the value of a for air is about 1-6x 10°, so that, when there are # positive and negative ions per cu. cm., the number of ions which recombine per second is 1°6x 107 §);2, this shows very markedly the influence of the electric charge in increasing the number of collisions between the particles, for the number of collisions in a second between 2x, uncharged molecules in a cu. cm. of air is 4x 1071027, which is only about 1/4,000 of the number of recombinations between the same number of ions. the ions like the molecules of the gas will in consequence of thermal agitation be moving with high velocities, and unless the distance between them is less than a certain value will drift apart and not combine. the condition that they should not separate if they are a distance d apart is that the kinetic energy due to their relative motion should be less than e?/d the work required to separate the ions to an infinite distance. the average kinetic energy due to their relative motion is equal to 8@ where §6 is the average kinetic energy of a molecule at the absolute tem- perature @; 8 is equal to 2-02 x 1076, hence the ions will separate unless 2:02x107 < e%/d. at o° c, @=273 and since e= 4-8 x10—\"® this condition shows that they will separate at this temperature unless d is less than 4-18 x10 centimetres. this is on the supposition that they are not subject to any influence other than their mutual attraction. as they move through the gas they may, if one or other of them when they are close together comes into collision with a molecule of the gas, lose some of the kinetic energy which would enable them to separate. assuming that collision with a molecule re- duces the kinetic energy due to relative motion to 66, the value it has for thermal equilibrium, it follows that to ensure combina- tion the collision with the molecules must occur when d, the distance between the ions, is less than ¢?/8@ and that every such collision leads to recombination. to find the rate at which the ions recombine we have to find the number of collisions between ions and molecules which occur in one second, only those col- lisions to be counted when an ion is within a distance d of one of opposite sign at the time of collision. it can be shown (see j.j. thomson, phil. mag.,47, p.337) that this number is equal to rdpp' v¥ u+ut(1 — aru\") ~(, re [z +=) ap —d 2d w! — =, =x, [x +=)) where a and a: are respectively the mean free paths of the positive and negative ion through the gas, u and u, the mean velocities of these ions due to their thermal agitation, p and p! the number of positive and negative ions per unit volume and d= e?/ bg. it follows from the definition that a the coefficient of recom- bination is given by the equation a = md? y¥ u?+u2 (1—ae!). when the pressure is low enough to make 2d/x and 2d/h; where small, then r—ww! = 3d (+; ) approximately, 1 see oe eae so that a = frd?v u+ur(¢ += ). 1 both 1/\\ and 1/a, are proportional to the pressure, so that at low pressures a varies as the pressure, which agrees with the results of thirkill’s experiments. when the pressure is so high that a/d and ai/d are small, w and w! are small and a= rde’v u?+u/7? approximately, so that a is independent of the pressure. gases, electrical properties of when the density is constant the value of a diminishes as the temperature increases. the connection between a and the ab- solute temperature t seems to be expressed with fair accuracy by the equation a= ci according to erikson, # is equal to 2-3, 2-42, 2:35 for hydrogen, air and cos respectively, while phillips’s experiments gave 2 = 2. diffusion of ions.—in addition to the loss of ions by recom- bination, there will be a loss due to the diffusion of the ions to the side of the vessel in which they are contained; when the ions strike against the walls they may lose their charges and thus cease to be ions. the loss of tons from this cause will be pro- portional to the surface of the vessel, while that due to recom- bination will be proportional to the volume, thus the ratio of the loss by diffusion to that by recombination will be greater in small vessels than in large. again the rate of diffusion is inversely proportional to the pressure while that of recombination is di- rectly proportional to it, thus the loss by diffusion will be rela- tively more important at low pressures than at high. large ions.—the ions we have been considcring are those pro- duced in dust-free gases by r6ntgen or cathode rays. in some cases, however, ions with very much lower mobilities are to be found in gases. thus langevin found in air from the top of the eiffel tower two types of ions, one consisting of ions of the kind we have been considering, with a mobility of about 1-5 cm./sec., the other of ions with a mobility of 1/3,000 cm./second. lons with mobilities of the same order as this second type may be produced by bubbling air through water, by passing air over phosphorus or by drawing air from the neighbourhood of flames. they are probably charged particles of dust of various kinds, held in suspension in gas which is exposed to some kind of ionising agent which gives a supply of ions of the first type; these settle on the particles of dust and form the slow ions. the number of these slow ions, when the gas is ina steady state, will depend only on the number of dust particles in the gas and will not be affected by the strength of the ionising agent. this follows from the principle that in the steady state the number of dust particles which acquire a positive charge must equal the number which lose such a charge. a positively electrified dust particle might lose its charge by meeting and coalescing with a negative small ion or by coalescing with a negatively electrified dust particle. these dust particles are, however, so sluggish in their move- ments that, unless the dust particles are enormously more numerous than the small ions, we may neglect the second source of loss in comparison with the first. thus if u is the number of uncharged dust particles in a cu. cm. of the gas, p and n the number of those with positive and negative charges respectively and p, », the number of positive and negative small ions, then the number of dust particles which acquire per second a positive charge will be aup and the numbcr losing such a charge by coalescing with a negative ion bp, where a and # are constants; hence for equilibrium aup = bpn similarly by considering the negatively charged particles we get a’un = p/np hence we see that the proportion between the charged and un- charged particles of dust depends only upon the ratio of p to n, and not upon the absolute magnitude of either of these quan- tities. thus, though it would take much longer to reach the steady state with a feeble source of ionisation than with a strong one, when that state was reached there would be as much dust electrified in one case as in the other. de broglie estimates that in this state about one-tenth of the particles would be electrified. relation between the potential difference and the current through an ionised gas.—we shall take the case of two infinite parallel metal plates maintained at different potentials and im- mersed in an ionised gas; the line at mght angles to these plates we shall take as the axis of x, it being evidently parallel to the direction of the electric force x. let m, m2 be respectively the number of positive and negative ions at the place fixed by the 143 co-ordinate x; 2% and 22 the velocities of these tons. the volume density of the electrification in the gas, if it is entirely due to the ions, is (#21—#2)e when c is the charge on an ion, hence lx | coe 4m — nae (1) dx if cis the current through a unit area of the gas e = e(nytti + ieite) (2) hence from (1) and (2) we have t rt te dx (3) e= — — am titi. 47 ute dx 3 t i mm dx nee = (4) tithe 40 tw+ite dx when things are in a steady state, neglecting any loss of ions by diffusion we have | (mus) = g—ainne (5) ee te) = g—anin (6) ar ote} = yj—aanine where ¢ is the number of ions produced per second in a cu. cm. of gas, and a is the coefficient of recombination; if ki, ke are the mobilities of the positive and negative ions respectively, then “y= hy tte = pox from equations (1), (5) and (6) we get d?x2 | ee a 8re(q—amn:)(~+~ ) and, substituting the values of ni and nz, we get (7) (ex? | eee a. he ax? ky dx? = sae{ —+1-- ss ee eae ld eat) (amie dx? re(i+;) 2 sere ga dx )( 8x dx ds no general solution of this equation has been obtained, but when ¢ is small compared with the saturation current gic, / being the distance between the plates, an approxi- mate solution is rep- resented by the graph in fig. 2. the force is practi- cally constant, and equal to 8 reer gf clkitke) except close to the electrode, where it increases; and as the mobility of the negative ion is greater than that of the positive the increase in the force will be greater at the cathode than at the anode. as the potential difference between the electrode increases, and the current approaches more nearly the satura- tion value, the flat part of the graph diminishes, and the graph for x takes the form given in fig. 3. when the potential differ- ence is so large that the current is nearly saturated, x is very approximately constant from one electrode to another. in one extremely important case, that in which the negative ions are electrons and have a mobility which may beregarded as infinite in comparison with that of the positive ions, equation (7) admits of integra- tion: for by putting fi/ke=o in 4 equation (8) it becomes fig. 3 dx? s8me*kogx* 87 dx = al be (8) the solution of this is 2 sas (142 i—& ~«) (9) gkz*e* kit 9 144 when x is the distance from the cathode and t the amount of negative electricity emitted by unit area of the cathode per unit time. this distribution of force is represented by the graph in fig. 4; the force at some distance from the cathode is equal to £ ey. ic and is thus proportional to the current; the force at the cath- ode itself is {1+ho(e—u)/ku}* times greater than this. the fall of potential between the electrodes is made up of two parts, one arising from the constant force; as this force is proportional to. this part of the potential fall will be proportional to ./ when electric force distance from cathode lis the distance between the electrodes, and may be represented by ad when a is a constant; the other part of the potential fall is that which occurs close to the cathode. we find from equation (9) that this is proportional to ? and does not depend upon /. thus, if v is the potential difference between the elec- trodes when a and b are constants v = ad+be. (10) h. a. wilson has shown that an equation of this type repre- sents the relation between the current and potential difference for conduction through ilames. in many cases the drop of potential at the cathode is much greater than the fall in the rest of the circuit; when this is so we see that the current is pro- portional to the square root of the potential difference. the value of b increases with the pressure and decreases with the amount of the ionisation. current from hot wires.—a case of great importance from its industrial application in hot wire valves is one where all the ions are negative and are emitted from the cathode. metal wires or plates raised to incandescence emit electrons, and if they are used as cathodes can transmit across a vacuum or gas at a low pressure very considerable currents. let the hot cathode be the plate x=o and let v be the po- tential at the point x, 2 the density of the negative ions at this point, and ¢ the current through unit area. if # is the velocity of the negative ion, we have 4 = 4mne. nue = et and dx? there are two cases to be considered; the first is when the hot plate is surrounded by gas of sufficient density to make the velocity of the ions proportional to the electric force; the second is when it is surrounded by a vacuum, and the motion of the ions is not affected by the gas. | lv os in the first case ua he, when kz is the mobility of the nega- ae tive ion, and the equation nue = is equivalent to ke dv dv ax dx dx . a) the solution cf this is dv\\? sri ; i) = bs x-+-c. | gases, electrical properties of therefore if v is the difference of potential between the anode and cathode, and / the distance between them 8 or lce ts) -¢] 127 k where c is the constant of integration. in consequence of thermal agitation some of the electrons near the plate will difluse backwards and hit the plate; if i» is the density of the electrons near the plate it follows from the kinctic theory of gases that the number hitting the plate in unit time is #oc/¥ 67, where c is the velocity of mean square, 7.e., 3mc = 2r@; m is the mass of an electron, @ the absolute temperature of the electrons and r the gas constant 1-35-+10°7®. if i is the current per unit area emitted by the plate, then in consequence of the diffusion there is a backward current enc! 67, so that t, the current going through the gas is given by the equation es (12) choc — = 6. vv 67 if #9 is the velocity acquired by the electrons near the plate under the electric force, .= cou, hence (13) so that, unless ¢ is small compared with i, #o will be comparable with c; in this case, however, the velocity of the ion is no longer proportional to the electric force so that equation (11) no longer holds. again, when the current approaches saturation, «¢/(i—¢) is large and therefore by (13) #o will be large compared with c. for the negative ion to acquire a velocity of this magnitude the electric field would have to be so strong that sparks would pass through the gas unless the pressure were very low. thus satura- tion currents from hot bodies are only obtainable at very low pressures. since the electric force when x=o is vc, w=kvc, and therefore by (13) m cc. 2 6rk? (i—2)? thus if the current is far from saturation, c will be negligible compared with 8rd/k. when c can be neglected, equation (12) gives _ ok v? gona? (14) thus the current is proportional to the square of the potential difference. a remarkable thing about this expression is that for these very small currents the intensity of the current is inde- pendent of the temperature of the plate, although, of course, the range of currents over which this formula is applicable is wider the higher the temperature. when the hot body is in a vacuum, we have, if the ions have no initial velocity, smu? = ve where m is the mass and e the charge on an ion; hence the equation nue =cis equivalent to (15) vi = qmev m/2e a solution of which ts ; v = (omt)i(m/2e)4x3. (16) hence, if v is the potential difference and / the distance between the electrodes 3 i ae) vi goose (17) onl? \\ m we see from equation (16) that the electric force vanishes at the cathode, and that the density of the negative electrification is as x-3; thus it is infinite when close to the cathode and dimin- ishes as the distance from the andde diminishes. the total quantity of electricity between the anode and cathode is pro- portional to (/?)§. we see again that for a given potential i= gases, electrical properties of difference the current does not depend on the temperature of the hot wire; this law only holds when the currents are less than the maximum currents which can pass between the electrodes. when the current approaches this value, the current instead of increasing as v? becomes independent of v, and the negative electricity between the electrodes diminishes as v increases. langmuir, who has made a very complete investigation of the currents from hot wires, finds that the expression (17) represents, with considerable accuracy, the relation between the current and rocce eo) ae cot [2a vette fe do ey yf sh ae eeeeeee eases oo ded mere role 11 rol woh ol skeet lcs let ated elie st tt peer ol bs or ff gc ld ge 7 se a a pl yl he ci a 6 ss se ll gh deo, uk int pc ae (rd pere bp d1 pe se ae aia fa sa ale ls oni fb bw hn mp il ey h+ (a ae ie aa pe pl $002} ar eels e fro 5”) ae . jel ae cea he : temperature fig. 5 potential over a wide range in the values of the currents. the curves in fig. 5, given by him, represent the relation between the current and potential for wires at different temperatures. they illustrate the point that a colder wire, until it is approaching the stage of saturation, gives as large a current as a hotter one, though the hotter one, of course, has a wider range of currents. tonisation by collision—the curve representing the relation between the currents through a gas ionised (say) by rentgen rays and the difference of a potential between the electrodes is found to be of the form already shown in fig. 1, where the or- dinates represent the currents and the abscissae the potential difference. the flat part represents the state of saturation when the potential difference is large enough to send all the ions pro- duced by the rays to the electrodes before they can recombine. when the potential difference is still further increased we see that a stage is reached when the current begins to increase with great rapidity with the potential difference, and reaches values much greater than could be attained by the ions produced by the rentgen rays. thus in addition tothe ions produced by the rays there must be other ions, and some other source of ionisation associated with the strong electric fields. now the processes going on in a gas while it is conveying an electric current are:— (1) the ionisation of the gas by the external agent—in this an electron is liberated from the molecule and the residue forms a positive ion; (2) the electron and the positive ion acquire energy under the action of the electric forces; (3) in many gases the electron finally unites with an uncharged molecule to form a negative ion. as the most noticeable change in the conditions when the intensity of the electric field increases is in the energy of the electrons and ions, it is natural to look to these as the source of the additional ionisation. we have, moreover, direct experimental evidence that rapidly moving electrons and ions are able to ionise a gas through which they are passing. hot 145 wires and metals exposed to ultra-violet light yield a supply of electrons which when they leave the metal have very little energy; by applying suitable electric fields these electrons can be endowed with definite amounts of energy and can then be sent through a gus from which all extraneous ionising agencies are shielded olf. when this is done it is found that, wher the energy of the electrons exceeds a certain critical value, depending upon the nature of the gas, the gas is ionised by the electrons, but no ionisation occurs when the energy of the electron falls below this limit. it is convenient to measure the energy of the electron in terms of the difference of electrical potential through which the electron has to fall in order to acquire this energy. the potential difference which would give to the electron the energy at which it begins to ionise the gas is called the ionising potential. the values of the ionising potential have been found for several gases, as will be seen from table iv. there is, however, consider- able discrepancy between the results obtained by different ob- servers. table iv. jontstng potentials in volts. stead & | franck | davis & | horton gosling | & hertz | goucher/& davies tate & | hughes foote the most obvious view to take of this ionisation by moving electrons is that the moving electron comes so near to an electron in a molecule of the gas that the latter receives from the col- lision enough energy to enable it to escape from the molecule and start as a free electron. if the electrons repel each other with forces varying inversely as the square of the distance between them, and if t is the energy of the moving electron, and d the length of the perpendicular from the electron in the molecule on the initial direction of motion of the moving electron, then the energy communicated to the electron in the molecule by its collision with the moving electron is equal to t ail nig bt e* , where e is the charge of electricity onan electron. this is on the supposition that the clectron is moving so rapidly that the time while it is in close proximity to the electron in the molecule is small compared with the time of vibration of that electron; if this time is comparable with the duration of the collision, the energy taken from the moving electron will be considerably less, and it will become vanishingly small when the duration of the collision is large compared with the time of vibration. the energy given to the electron in the molecule does not increase indefinitely with that of the moving electron, for it vanishes when t is infinite as well as when t is zero; it has the maximum value when t=e?/d. in order that the electron in the mole- cule should receive an amount of energy q t e(t/q—1 oa ge gee nord co, a de ake \"a | if q is the ionising potential, d? must be less than the value given by this expression. if 2 is the number of electrons in unit volume of the gas, and if the spheres with radius d described round the different electrons do not overlap, the probability that the moving clectrons should come within this distance of one of them, when moving through a distance ax, is w7d*ax, or | we ae 146 the coefficient of ax is the number of ions made per unit path by a moving electron with encrgy t. the maximum is when t=20q. expcriments on ionisation by moving electrons have been made by kessel (ann. der phys. 37, p.406) and by mayer (ibid. 45, p. 1), who found that the maximum ionisation per unit path occurred when the energy of the moving electron was in the neighbourhood of 200 volts. mayer’s results are 125 for hydro- gen, 130 for air, and 140 for carbon dioxide. these numbers are much greater than twice the potential at which the jonisation begins, as this potential is of the order of 11 volts. it must be remembered, however, that though there may be some electrons in the atom which can be ejected by 11-volt electrons, there may be other electrons of different types which require more energy for their expulsion, so that, as the energy of the moving electrons increases beyond the energy required to liberate these electrons, fresh sources of detachable electrons will be trapped, and these may more than counterbalance the falling off in the tonisation of the more easily detached electrons. again, some of the electrons ejected by the primary electrons may have enough energy to ionise on their own account; the total ionisation may thus be increased by ionisation due to the secondary electrons, and also by radiation excited by the impact of the primary electrons against the molecules of the gas. when, as in the case of cathode rays in highly exhausted tubes or in that of the @ rays from radioactive substances, ‘t is very large compared with q, the number of ions produced per unit path is #me’/qt, and so varies inversely as the energy of the moving electrons. the experiments of glasson on ionisation by cathode rays, and of durack on that by f-particles, seem to be in accordance with this result. scatiering of electrons ——the researches of lenard and ramsauer on the scattering of electrons in their passage through gases, have shown that the relation between the scattering produced by different gases depends to a great extent upon the velocity of the electrons which are scattered. if this velocity is small the scattering by most gases is approximately that which would be produced by impenetrable atoms of the size indicated by the kinetic theory of gases and derived from the measurements of viscosity of gases and so on, there is no clear connection between the scattering produced by a gas and the number of electrons in its atoms. with high speed electrons the laws are much simpler and show proportionality between the scattering and the number of electrons in the atom. we should expect these results on the theory just given. for if 28 is the angle through which the relative velocity of the electron and the atom is deflected by the collision sin? @ = when t is the kinetic energy dut to the relative motion of the electron and the atom. let us put da - then if t is expressed as v volts c = 14-4x10°3/v and sin? ? = : ig we see from this that 9 will not be small unless d is considerably greater than c. thus we may regard each electron in the atom as deflecting through considerable angles all electrons which pass within a distance c. they may, as far as collisions are concerned, be regarded as producing much the same effect as solid spheres of radius c. now when the energy of the electrons is represented by 1 v., c is 14:41o-8 centimetres. this is much greater than the distance between the electrons in the atom; thus the fields of influence of the different electrons in an atom will overlap, so that an increase in the number of electrons in the atom will not produce much effect on the scattering. on the other hand when gases, electrical properties of t the energy of the electrons is so large that c, which is inversely proportional to t, becomes less than the distance between the electrons in the atom, the fields of influence of the different electrons will not overlap and each electron will provide an addi- tional source of scattering, so that in this case the scattering will be proportional to the number of electrons in the atom. with very slow electrons the scattering in most gases is in- dependent of the speed but for argon and the heavier inert gases ramsauer has made the very interesting «liscovery that very slow electrons are not scattered so much as those moving somewhat faster, it is asif the atoms of these elements were more permeable to slow electrons than to fast. tonisation by moving ions—when the moving systems are ions instead of electrons, the collision between them and the electrons are collisions between masses of very different magnitudes, and in consequence a very much smaller fraction of the energy of the moving body becomes transferred to the electron than when the colliding bodies have equal masses. the amount of energy transferred to the electron when the moving body has a mass m; is equal to:— 4m\\m2 ak (m+ m2)? 40t*/ m, y ek? m:+ m: when m2 is the mass of the electron and e the charge on the moving body. when, as in the case of the collision between an ion and an electron, m2 is very small compared with mi, this becomes 4ma t ; my 4a°t? m-/ eh? my | thus, if q is the ionising potential, the minimum value of t, ; m which will communicate this energy to the electron, is are: 2 for the smallest possible ion, an atom of hydrogen, m\\/m:= 1,700, so that the minimum energy that will enable an ion to ionise a gas by knocking out an electron from a molecule is equal to 425q. q for many gases is about ro v.; thus a positive ion must have at least energy represented by 4,250 v. to ionise the gas. with more massive ions the energy required for ionisation would be still greater. an ion with a mass equal to that of a molecule of oxygen would not ionise unless its energy were greater than 136,000 volts. thus any ionisation by ions taking place in discharge tubes where the potential difference is in general much less than this value must be due to ions of the lighter elements, hydrogen or helium. if the ion came into collision with the atom instead of with one of its electrons, it could, since its mass is comparable with that of the atom, give up to this a large fraction of its energy, a very much larger fraction than it is able to give to an electron. inas- much as it requires less work to dissociate a molecule into neu- tral atoms than to dissociate it into positively and negatively electrified ions, the result of such a collision is more likely to be the production of neutral atoms than of electrified ions. an ion is, however, a much more complex thing than the simple charge of electricity which has, in the preceding considera- tions, been taken to represent the forces it exerts; and it may be that some strongly electronegative ions have such a strong attraction for an electron that when they pass through the molecule of a more electropositive element they are able to capture one of its electrons and carry it away with them. this type of ionisation would differ from the ordinary type, inasmuch as in it the electron is never free; this type produces negative ions, the other type negative electrons. it is evident from the preceding considerations that except in very intense fields it must be the electrons and not the ions which produce ionisation by collision. let us consider what are the chances of an electron acquiring sufficient energy in a uni- form electric field; if the electron moved freety under the electric force x for a distance / it would acquire xe/ units of energy. gases, electrical properties of the electron in its course through the gas will come into collision with other bodies; its path will be deflected, possibly reversed, and in moving against the electric field it may lose all the energy it had previously acquired. thus a collision of this type may destroy any ionising power given to the electron by the electric field before the collision. let a be the average distance passed over by an electron between two collisions; then the chance of an electron moving through a distance j without a collision is e—, but if it moves through a distance? it will acquire energy = xcel, hence the chance of an electron acquiring energy equal to or greater than t is t e *€% and the chance that it should acquire energy between t a 1 / -= and t+dt is ma xx ar, if it possess this amount of energy the chance that it makes one ion per centimetre of path is c| nr (tiq—1); hence a, the chance that an electron should make one pair of ions per centimetre of path, is given by the equation t ody - yy dt = 4 plaids xer pah), pam a = te fi. tag : )(t/q r) 72 this may be written niet a= oo f(e@) f(6) = fy (2-2 )as since \\ for the same gas is inversely proportioned to the pres- sure ~, a will be of the form xf(x/p) where f(x/p) denotes a function of x/p; and since » is proportional to the number of molecules per unit volume, a may be written as pf(x/p). when the spheres described round the electrons with radius d h t_,) do not overlap, # will also be proportional to the number of clec- trons in the molecule. the greatest value of d is e?/2q; hence if d, the distance between two electrons, is greater than e?/2q there can be no overlapping; if d is less than this quantity, there may be overlapping; since the value of d diminishes as the kinetic energy of the electron increases, » for very fast electrons will be proportional to the number of electrons in the molecule. some of the electrons will by adhesion to a neutral molecule become negative lons. let the chance of an electron doing so while passing over 1 cm. be yp. if n be the number of electrons where 6 = q/xexx and per c.c.at a place fixed by the co-ordinate x, then + (nu) = dx rate of increase of number of electrons per c.c., where u is the velocity of the electron parallel to x. the number of electrons passing through the unit of area in unit time is nu. the new electrons produced by the passage of them through the unit volume is nua, while nuy? will disap- pear; hence:— an, lt a tg, ror nu yp) +¢ (19) where g is the ionisation due to external sources; when things are in a steady state dn/df=o, and the solution of the equation, when the electric field may be taken as constant from one elec- trode to another, is:— nu = cele-19)x__4 a— yp most of the experiments on this subject have been made with- out external ionisation; a supply of electrons has been obtained from the cathode, either by raising it to incandescence or by ex- posing it to ultra-violet light. in such cases q=o, and nu = wg'(2— yp)x (20) where ty is the number of electrons emitted in unit time from the cathode. townsend, and townsend and kirby have determined 147 the value of a—y? for various gases and over a considerable range of pressure. a series of these values for air are given in table v. table v. x = volts per pressure (mm.) done nn opn odor p* ~ wo 1°3 2-0 2:8 3°4 3°8 4°5 5:0 5-4 5:8 6:2 comi gg ; . wmed daheotinb ew... an) it will be seen that, when x is given, the increase in the number of electrons reaches a maximum for a particular pressure. from gencral reasoning this must be so, for if p=o there will be no collisions to make fresh electrons, and if # is infinite the free path of the electrons will be so small that they cannot acquire ee : ; x sufficient energy to ionise the gas. since a is of the form pf (): and y does not depend upon p,a—y? will be a maximum when x my xx neal fen ee ig) v o(s) rc or when { () —y= f! ga) where {'(x/p) denotes the differ- ential cocficient of {(x/p) with respect to x/p. this equation determines x/p; hence the critical pressure will be proportional to the electric force. at this critical pressure xex bears to qa ratio which depends upon the way in which the chance of an electron ionising by a collision depends upon the energy of the electron. if, for example, the chance were independent of this energy, provided the energy were greater than q, the maximum current would be when xea=q; this relation would not hold for other and more probable laws connecting ionising power with the energy, but we should expect that for any such law the ratio of xex to q would neither be very large nor very small. since the electrons cannot begin to ionise until their energy is equal to q, and to attain this energy they must pass through a distance q.'xe, it is clear that we ought in such an equation as (20) to write x—q/xe in place of x. if vis the potential difference between the plates, x=v/d, so that x-q/xe=x—dq/v if q is measured in volts. thus in finding the current between two electrodes we must, if we use equation (20), write a(x-\\) instead of d. partsch (verh. d. deutsch. phys. gesell., 14, p. 60) has shown that theory and experiment agree better by this change. tonisation due to radiation —when rays of ultra-violet light of very small wave length or rentgen rays which are vibrations with yet smaller wave length pass through a gas, the gas is ionised. the application of c. t. r. wilson’s expansion method (supra) to this case shows that the effect of the radiation is to expel from some of the gas an electron moving at a high speed. this electron ionises by collision the gas through which it passes and the majority of the ions are produced in this way. this is proved by the fact that on the wilson photographs (sce fig. 18) the drops of water which denote the presence of the ions are found to be arranged on lines starting from a molecule of the gas. for the ionisation to take place there must be some process going on by which the energy of the radiation can be concentrated on the minute volume occupied by an electron. the energy with which the electron is expelled from the molecule depends on the fre- quency of the radiation but not upon its intensity. thus if the source of radiation be moved further away from the gas the ' number of high speed electrons ejected is diminished but the 148 speed of those which are ejected is not affected. this result has led to the quantum theory of radiation, which supposes that radiant energy is made up into units, or at any rate is only communicable by such units, the unit of energy is proportional to v the frequency of the radiation and equal to fv where / is a constant called planck’s constant. on the c.g.s. system of units it is equal to 6:55x10-*’. since to expel an electron from a molecule requires the expenditure of ve units of work when v is the ionising potential, if the electron is expelled by the radiation the energy available must be greater than ve, hence »v the fre- quency of the light must be greater than the value v given by the equation hv = ve. for example if the ionising potential of helium is 22-7 volts, helium will not be ionised by radiation unless the wave length of the radiation is less than 540 angstrom units. it has been shown by barkla that when the radiation is wholly absorbed in a gas the number of high speed electrons ejected is independent of the nature of the gas. thus we may regard the production of a high speed electron as involving the destruction of a unit of radiant energy. the connection between the wave length of the radiation and the amount of ionisation produced in gases of definitely chemical constitution is a most vital and important one which unfortu- nately has not received the attention it deserves. one most important question is whether the ionisation is an additive property, or not. if the chemical composition of the gas is repre- sented by a:b,c. where a,b,c, are symbols representing chemical elements, and x, y,2, are the number of atoms of a, b,c, in the molecule, can the ionisation per molecule, when radiation of known frequency passes through the gas, be represented by x(a)-+y(b)+-2(c) where (a), (b), (c), are constants appertaining to the elements a, b, c, respectively? the evidence on the whole seems to be in favour of such a connection but the tests have not been very severe. the amount of ionisation due to radiation varies enormously with the nature of the gas. thus the ionisation in hydrogen is so small that some observers consider that such as is observed is due to a trace of impurity. it is not comparable with that which occurs in gases like chlorine, sulphur, bromine, iodine and mercury vapours. it has been found that yu the absorption of radiation of wave-length x estimated per molecule of the absorbing substance can be represented by formulae of the type p= cn where n is the atomic number of the absorbing substance and c aconstant. the values of p given by different observers range from 2-5 to 4 and of g from 2-5 to 3. values such as these indicate that the absorption and presumably the ionisation increase rapidly with the molecular weight. thus if we take p=3 the absorption of the rays by an atom of iodine would be about 1,000 times that by an atom of carbon. for the same substance and different kinds of radiation, the ionisation increases as the cube of the wave length. formulae of the type cn’? hold over only a limited range of frequencies; the values of p and gq appear to change when the wavelength of the radiation passes through x a value corresponding to that haiti of a characteristic radiation of the absorbing substance. the graph representing the relation between the wave ane and the absorption is of the type shown in fig. 6. | it will be noticed that this curve difiers very materially from gases, electrical properties of the graph corresponding to the visible part of the spectrum. in this part of the spectrum we may have very intense absorption of light of a particular wave length, for example, the absorption by mercury vapour of the line 2536 is so strong that the light cannot pass through more than a few millimetres of the vapour at the pressure of one hundredth of a millimetre of mercury, and yet there is practically no absorption for lines whose wave length is cither greater or less than 2536 by 1 angstrom unit. with radiation of the ionising type, on the other hand, though the absorption comes in abruptly when the wave length falls below a critical value, it does not fall away abruptly when the wave length is still further diminished but decays according to the law. the application of this law leads us to expect that radiation between a certain range of wave lengths say from 20 to 250 angstrom units would be exceedingly powerful ionising agents. for the characteristic radiation given out by zinc whose wave length is 1-4 a, u has been found to have the values given in the second column of table vi., the value of uw calcu- lated by the 3 law for radiation of the wave length 248 a, for which the quantum is 50 v., is given in the third column. table vi. values of p by the 8 law. substance ha=ry4a a= 248 a carbon 4:26 1:8 x10? air at o°c. & & 760 mm. g91x104%, 40x10 meg ; i ; 59 2:6x 108 | al reg ; : 105 4:6 x 108 | fe ; ; . ; a ne 76x10? cu a 490 2:2x10! | since the radiation falls to 1/e of its value after passing through a distance equal to 1/u we see that the radiation with the longer wave length would be practically completely absorbed by a layer of al or mg one molecule thick or by a layer of air one centimetre thick at a pressure considerably less than that due to a tenth of a millimetre of mercury. j. j. thomson has shown by measuring the photo-electric effects produced inside the discharge tube by the radiation produced by the passage of the electric discharge through the tube that this radiation includes not only the visible part of the spectrum where the energy in the quantum is only some two or three volts, but soft rentgen rays having quanta between 20and roo volts. these rays as we have just seen will be greedily absorbed by the gas and produce intense ionisa- tion. a large part of the ionisation in the tube is probably due to these rays which are excited by the passage of cathode and positive rays through the gas and by their incidence on the elec- trodes and on the walls of the discharge tube. spark discharge—the production of ions by moving electrons will not by itself explain why a current of electricity can be maintained through a gas by an electric field when all other sources of lonisation are excluded. the electrons are continually being driven towards the anode, and unless there is some source of supply near the cathode the ionisation and therefore the current will rapidly come to an end. one ‘way in which the electrons could be supplied by the action of the electric field would be by the positive ions which strike against the cathode communicating so much energy to the electrode that it is raised to incandescence. since an incandescent metal gives out large quantities of electrons there will be a continuous supply of electrons from the cathode, which will ionise the gas and produce fresh positive ions to strike against the cathode and keep it hot. this is what happens in the arc discharge when the cathode is kept in a state of incandescence by the discharge. in this case there is a large amount of energy put into the arc. there are, however, other forms of continuous discharge where the cathode does not become incandescent, so that there must be other ways in which the supply of electrons is maintained. from what we know about ions there are several ways in which this might occur. it has been found by experiment—i[fiichtbauer, ann. der phys., 23, p. 301 (1907); saxen, anz. der phys., 38, p. 319 (1012); baerwald, ann. der phys., 41, p. 643 (1913); 42, p. 1207 (1913) ]— that electrons are emitted from metals when these are bombarded by high speed positive ions even though the metal is not raised to gases, electrical properties of incandescence. according to baerwald the emission of electrons from metals bombarded by positive hydrogen atoms does not become appreciable until these have an amount of energy ex- ceeding that represented by 900 volts. again positive ions ionise a gas through which they pass. this was shown by mcclelland, who found that the relation between the potential difference and the current from a hot wire anode surrounded by gas at low pressure was represented by a curve like that shown in fig. 7. the hot wire furnishes positive ions as well as negative ones, and ae i pepe 120 160 200 240 280 320 360 fig. 7 the curve shows that fresh ions are formed when the potential difference is greater than about 200 volts. this is a much greater potential difference than that needed to produce ionisation by electrons, but it is smaller than would be expected by the con- siderations given above. as it requires less work to eject an electron from a metal than from a molecule, we should expect that if 200 v. ions could eject clectrons from a gas through which they pass they would be able to do so from a metal against which they strike, but from baerwald’s experiments much more energy than 200 v. is required for this purpose. in mcclelland’s experi- ments the ionisation might have been into positive and negative ions rather than into positive ions and electrons; before the negative ions could be efficient for ionisation by collision they would have to undergo further dissociation into electrons and uncharged molecules. curves similar to that in fig. 6 have also been obtained by o. w. richardson. pawlow (prec. roy. soc., a. go, p. 398) and also franck and e. v. bahr (werk. d. deutsch. phys. ges., 16, p. §7, 1914) came to the conclusion from their experiments, that ionisation was produced by positive ions even when their energy did not exceed a few volts; indeed they could not get any evidence of a minimum to the ionising voltage. ilor- ton and davies (proc. roy. soc., 05, p. 333) could not detect any ionisation in a gas by positive helium ions when the energy was due to 200 volts. they ascribe the ionisation observed by pawlow, bahr and franck to photo-electric effects; they consider, however, that positive helium ions can liberate electrons from a metal against which they strike if their energy exceeds 20 volts. baerwald considers that it requires an energy measured by goo v. before positive ions can liberate electrons from metals. there are thus at least four methods by which the supply of electrons near the cathode necessary to maintain the discharge can be obtained. the gas near the cathode may be ionised by positive ions or by radiation, or the cathode itself may emit electrons under the impact of positive ions or by the incidence of radiation. when the gas is at a low pressure, the appearance of the dis- charge has well-marked characteristics which may throw light on the method by which the electrons are produced and the place from which they start. the discharge near the cathode is repre- sented in fig. 8; in contact with the cathode there is a velvety glow, then a space comparatively dark called the cathode dark space; this joins on to a brightly luminous region called the negative glow; passing through this region, and making them- selves evident by the luminosity they excite when they strike against the glass wall of the vessel in which the gas is contained, are the cathode rays. these have been shown to be electrons moving with high velocity. these electrons have been liberated by the action of the electric field and have acquired their velocity under the action of that field. the velocity of the cathode rays has been 149 measured, and it has been found that the majority of them have the same velocity. this shows that they must have all fallen through the samc potential. they would do this if they all started from the cathode itself, but if they had originated by the ionisation of the gas in the dark space in front of the cathode some would have started from one place and some from another, and they would have acquired different velocities. this is strong evidence in favour of the cathode itself being the primary source of the electrons which maintain the discharge. when a supply of electrons 1s produced by processes taking place at the cathode, ionisation by collisions of electrons with the molecules of the gas is sufficient to maintain the discharge through the interval between the negative glow and the anode. this interval, as will be seen from fig. 8, is made up of a short part next the negative glow in which there is comparatively little light, called the faraday dark space, and then a long uniform portion reach- ing right up to the anode. unless the pressure is very low or the spark very short this portion, which is called the positive column, forms by far the larger part of the discharge. the discharge here will be maintained if the rate at which electrons are produced by collision is equal to the number lost by recom- bination. when this is the case, equation (19) gives a=y¥f, or, since a is of the form +f (i thus xedk=cq where ¢ is a quantity which does not depend upon the pressure or strength of the field; as x is inversely proportional to the pressure, this equation is equivalent to x=cp, when c is a quantity which will depend on the nature of the gas and pos- sibly on the intensity of the current. if ? is the length of the positive column the difference in potential between the anode and the end of the positive column next the cathode is /x, i.e., cpl. between the cathode itself and the negative glow there is a fall of potential, called the cathode potential fall, which, when the current carried by the discharge is not large, is independent of the current and the pressure of the gas; it depends upon the nature of the gas and the material of which the electrodes are made. if vo is the cathode fall, then (neglecting the change in potential in the negative glow and the faraday dark space, which has been found by experiments to be very small) v, the potential difference between the anode and cathode, will be given by the equation = votcalp (21) it is assumed that the length of the spark is greater than that of the dark space d; at pressures comparable with that of the atmosphere, d is a very small fraction of a millimetre, but at the low pressures which can easily be obtained in highly exhausted vessels d may be several centimetres. it is to be noticed that v is a linear function of 7p, and / is proportional to the mass of gas between the electrodes; hence as long as the mass of gas between the electrodes remains unaltered the potential difference required to maintain the spark will be constant. this law, which was discovered by paschen in 1889 as the result of a long series of experiments, is known as “ paschen’s law.” it has been found to be in agreement with the very numerous investigations which have been made on the potential difference required to produce a discharge in an approximately uniform electric field such as that which exists between two slightly curved electrodes. the relation (21) does not give any indication of the relation between the potential difference and the spark length when the 150 latter is exceedingly small. when the spark length falls below a critical value which is inversely proportional to the pressure, and which in air at atmospheric pressure is about -or mm., the spark potential increases rapidly as the spark length diminishes; this was first observed by peace. a simple way of demonstrating it is to use slightly curved electrodes and to observe the path of the spark as these are brought closer together. until the elec- trodes get very close together the spark passes along the shortest line between them, but as they approach each other a stage is reached where the spark no longer passes along the shortest line 2000 4 ae eel s“ ie en oe = ae ial 2d s 600 5 [a a ow ms 400 2 a @ s oe ee ss 200 3 o = & . e product of pressure ond yee: between ae fig. 9 but goes to one side, taking a longer path, showing that it is easier to produce a long spark than a short one. the relation between the potential difference and the spark length for several gases has been determined by carr, who finds that paschen’s law that the potential difference depends only on #/ is also true for very short sparks; paschen’s own experiments were made with sparks considerably longer than the critical value. fig. 9 repre- sents carr’s results for the relation between v and p/. the results of carr’s and strutt’s (lord rayleigh’s) experiments for the minimum spark potential, and the value of ~/, at which it occurs, are given in the following table:— table vii. minimum spark potential in pl volts 4 | ne 341 s! 5:7 nitrogen . 251.5 6-7 oxygen : : ; 455 c bs hydrogen g wel we ca a. { 302-308 s 14-4 278 carbonic acid 419 c 51 sulphur dioxide 457°c 253 nitrous oxide 5 sulphuretted hydrogen 6 acetylene is helium \\s=by strutt; c=by ce the curves are very flat in the neighbourhood of the minima, so that the critical values of p/ may be subject to considerable errors. strutt found that even very small traces of impurity produced very large increases in the values of the minimum spark potential in nitrogen and helium; these are gases where, as we have seen, such traces produce large diminutions in the mobility of the negative ion. the existence of a minimum for the spark potential and a critical spark length follows from the view that the self-sustained discharge is maintained by radiation produced by electrons or positive particles. the condition for the self- sustained discharge is that an electron in its passage between the electrodes exerts some ionisation effect which produces another electron to succeed it. if v is the potential between the plates, the energy given to the electron is ve. let the amount of this converted into radiation be 6ve;if uw is the coefficient of absorp- tion, d the distance between the plates, then when d is small the number of ions produced by the electron will be budve. if the gases, electrical properties of electron is to provide a succession this must be equal to unity so that i ~ bed so that the potential difference varies inversely as 1/d and is infinite when d vanishes. since wis proportional to the pressure of the gas we see that v will depend upon fd so that paschen’s law is fulfilled. the mechanism we have hitherto considered involves the ionisation of the gas between the electrodes, and no spark could pass across a vacuum. there are, however, other methods by which a discharge might do this. for suppose there was a stray electron between two parallel electrodes in a vacuum; then under the action of the electric field it would be driven against the anode; by the impact rintgen radiation would be generated, which would fall on the cathode, and if it were intense enough to liberate one electron from the cathode the original electron would be replaced and the passage of negative electricity from the cathode to the anode would be repeated. from these con- siderations it is probable that even the highest vacuum would not act as a perfect insulator for very intense fields. the linear relation v=vote/p has beeri obtained on the assumption that the direction of the electric force was the same in all parts of the field; this is only true when the dimensions of the electrodes are large compared with the distance between them. the potential difference required to produce a spark of a particular length depends upon the size of the electrodes between which the spark passes, and is not a linear function of /, where # is the pressure and 7 the spark length, unless / is small compared with the linear dimensions of the electrodes. if these are spheres, the spark potential will depend upon their radu, and for small spheres may be considerably less than for large ones. thus, for example, the spark potential in air for a five centimetre spark is 26,000 v. for electrodes -5 cm. in diameter, and 105,000 v. when the diameter of the electrodes is 5 centi- metres. in this connection it may be noted that, if the electric field is sufficiently intense at any place to produce there a local supply of ions, these may redistribute themselves between the electrodes, and by their electrostatic action produce a change in the dis- tribution of the electric force more favourable to the passage of the spark than that prior to the production of the ions. to illustrate this, take the very simple case when the electrodes are two parallel plates: if there are any ions available these may distribute themselves so that the force between the plates is no longer uniform. thus let us suppose that there are enough positive ions to congregate round the cathode in sufficient num- bers to produce within the distance of the “ critical] spark length ” or thickness of the cathode dark space, a difference of potential equal to the minimum spark potential. this would ensure a continual emission of electrons from a place in front of the cathode, and even though the electric field from this place to the anode were too feeble to give an electron enough energy to ionise the gas, the electrons coming from the cathode would be able to carry a small current, though this part of the discharge might not be luminous. the ions here would be all of one sign, so that the electric force will increase up to the anode. if the current is gradually increased, the place where the clectric force will first rise to the value necessary to make the electrons ionise will be close to the anode. when this occurs a supply of positive ions will start from the anode and move towards the cathode, accom- panied by luminosity close to the anode and very faint luminosity through the rest of the tube. the introduction of the positive ions into the region between the anode and cathode will diminish the retarding effect of the negative space charge which existed in this region, so that the current will increase. this increase in current will again increase the ionisation at the ancde, and thus the supply of positive ions. in this way there might be a supply of electrons coming from the cathode, and of positive ions from close to the anode, which will maintain the current in spite of the fact that between these places there is a region where the electric gases, electrical properties of force is below that required to produce ionisation by collision, and the potential difference between the electrodes less than that calculated on the supposition that the electric force was uniform from one to the other. we should expect from these considera- tions that, if the electric force at any point were intense enough to produce ionisation by collision, some discharge would take place. russell (phil. mag., 6, xi., p. 237) states that the results of the different experiments made on the potential difference required to produce sparks of various lengths between spherical electrodes of various radii are in good agreement with the rule that the dis- charge takes place in air at atmospheric pressure if the electric force at any point in the field before discharge begins is as great as 37,000 vv. per centimetre. this value agrees well with that required to make electrons produce, in airat atmospheric pressure, other ions by collisions. the curious lag observed by warburg between the application of the potential diflerence and the passage of the spark, which when the applied potential is only a very little greater than that required to produce the spark may amount to several seconds, is naturally explained as the time needed by the ions to dis- tribute themselves so as to produce the distribution of potential required for the discharge. the discharge of electricity from points affords a good illus- tration of the preceding considerations. suppose that the elec- trodes are a needle point and a plane. when the discharge first begins the only place where any light is to be seen is close to the point; the current between the electrodes is very small; as the potential difference increases a stage is reached where light begins to appear close to the plate, the space between the point and plate being quite dark. this stage is marked by a large increase in the current. with further increase in current the luminosity extends into the gas and ultimately stretches from one electrode to another. the potential required to start the discharge is less where the point is negative than when it is positive. this is what might be expected, for to maintain the discharge from the negative point there must be (1) ionisation of the gas by the outgoing elec- trons, and (2) liberation of electrons by the incoming positive ions, while when the point is positive there must be (1) ionisation of the gas by outgoing positive ions, and (2) liberation of positive ions by the impact of incoming electrons; as the process is not the same as for the negative point we should expect that there woulil be a difference between the potentials. it is not only the potential difference which is affected but the type of discharge. this can be shown by allowing the point discharge to pass in the neighbourhood of a photographic plate. beautiful figures are found on developing the plate, and the character of these is different according as the point is positive or negative. figures ro and 11 represent discharges from positive and negative points respectively. the discharge from a negative point is in some gases very much influenced by the purity of the gas; thus warburg found that the discharge from a negative point in nitrogen increased about 50 times by removing the last trace of oxygen from the nitrogen, though this had little or no effect upon the discharge from a 151 positive point. this can be accounted for by the discovery of franck and hertz that in very pure nitrogen the electron does not unite with the molecule and become a negative ion and has therefore a very high mobility. this is true for the inert gases as well as for nitrogen, and przibram has shown that the differ- ence between the discharges from positive and negative points is exceptionally large in these gases. electrical wind.—the electrified ions’ starting from the point in a point discharge sets the gas in the neighbourhood of the point in motion producing a current of air, called the “ electrical wind.” the momentum gained by the air is lost by the point, so that there is a backward force acting on the point, which has often been measured. this force, as well as the electrical wind, is smaller when the point is negative than when it is positive; this difference is especially marked °t pressures low enough to make the negative ion have an abnormally large mobility. relation between potential difference and current.—the po- tential difference required to maintain a discharge will depend upon the current passing in the discharge. the relation between the current and the potential difference for discharge through gases is often a very complicated one. we should expect that this would be so, for in the spark discharge, for example, the potential difference is made up of the cathode fall of potential (this increases with the current when the current is large) and a uniform force along the rest of the discharge, and this force in many cases diminishes as the current increases. thus whether increases of current produce an increase or decrease in the po- tential difference will depend on the relative contributions of these two parts. a curve whose ordinates are the potential difference between the electrodes and the abscissae the current through the gas, is called the “ characteristic curve’ for the discharge (see fig. 12). from such a curve it is possible to find the current sent through a gas by a given external electromotive force. suppose that the current sent through a gas by a battery of cells of electromotive force ep is required. if r is the resistance of the curves connect- ing the battery with the electrodes in the gas, then eo— reis the potential difference between the electrodes in the gas, and one relation between this potential v and the current is represented by the straight line v=e)—re. the other relation 1s that repre- sented by the characteristic curve; the values of the current through the gas and the potential difference between the elec- trodes will be determined by the points of intersection of this straight line and the characteristic curve. unless the straight line cuts the curve there can be no discharge through the gas; potential difference ou current on the other hand, the straight line may cut the characteristic curve in more than one point, indicating that there is more than one type of discharge. some of these types may, however, be unstable and thus impossible to realise. thus, for example, if the current is increased by 6 the difference of potential given by the battery between the electrodes is diminished by rec; 152 if v is the potential difference between the electrodes required to send a current « through the gas, then when the current is * * s . 4 . dv increased by 6, the increase in the potential required is a, ot l dv thus unless 7, is less than —r&, or — (“+ r) be be positive, l the diminished potential supplied by the battery will not be sufficient to maintain the increase in the current; this increase will stop, the current will return to its original value, and the . es o d s e es s discharge will be stable; thus if r+\\ is positive the discharge ge tg 1v : : will be stable. if, however, r+—- is negative the fall in po- lo? tential required to maintain the increased current is so great that, in spite of the diminution of the potential difference supplied by the battery, the residue is great enough to maintain the increased current, the increase in the current will continue, and the dis- charge will be unstable. thus the condition for stability is that rs should be positive, a result first given by kaufman. this t result is equivalent to the condition that for stability the straight line must, at the point where it cuts the characteristic curve, fall more steeply than the tangent to the curve at that point. thus if apqb, fig. 12, the characteristic curve and the right line cut at pq, the type of discharge represented by p is unstable, and that by q stable. keeping the electromotive force at the battery constant and increasing the resistance will make the straightline steeper, and q will move tothe left and the current through the tube will decrease; when the line gets so steep that it touches the curve at s, the minimum value of the current consistent with the maintenance of this type of discharge by the electromotive force supplied by the battery will be reached, and any further diminu- tion of the current will result in the extinction of this type of discharge. it is a well-known fact that the existence of most types of luminous discharges requires the current to be above a certain critical value which depends upon the external force. the elec- tric arc is perhaps the most familiar example of this. as the characteristic curve for the arc discharge is a rectangular hyper- bc b bola represented by the equation v=a-+-; we can easily show l that if the external electric force is e, the maximum resistance which can be introduced into the circuit without extinguishing the arc is (e—2a)\"/44, and the smallest current compatible with the existence of the arc 25/(f.-a). for any stable type of dis- charge we see that an increase in the external clectromotive force will result in an increase of current; at a point corresponding to an unstable condition it produces a diminution. structure of the dischurge—the structure of the discharge at atmospheric pressure is on so fine a scale that its details can only be made out with difliculty; as the pressure is reduced the scale gets larger and larger, until, when the pressure is reduced to that due to a millimetre or so of mercury, the details of the structure become very conspicuous. the appearance of the discharge at such a pressure is shown in fig. 13, and we see that it is built up of several constituents of very different types. we have already, when considering the spark discharge, given a general description of some of them; there are, however, some features which require further discussion. starting from the cathode, we find a thin layer of luminous gas, the colour of which depends on the kind of gas through which the discharge is passing. in most gases the light appears to reach gases, electrical properties of right up to the cathode, but in helium aston has shown it is separated from it by an exceedingly thin dark space. this lu- minous layer is sometimes called “‘ goldstein’s first layer’; next to this we have a region where there is comparatively little luminosity called ‘‘ crookes’ dark space,” the boundary of this space being approximately the surface traced out by normals to the surface of the cathode of constant length. the thickness of the dark space, which is of the order of the critical spark length, depends upon the pressure of the gas, varying approximately as the reciprocal of the pressure; for air at the pressure of 1 mm. of mercury the thickness of the dark space is about 2 mm. so that at atmospheric pressure the thickness would not be much more than about 1/400 of a millimetre. if the pressure remains con- stant and the current through the tube is increased, the thickness of the dark space remains unaltered until the current is large cnough to cover the whole of the cathode with the luminous glow; after this stage 1s reached any further increase in the current causes a diminution in the thickness of the dark space. starting from the boundary of the dark space there is a brightly luminous region called “‘ the negative glow.” the function of the parts of the discharge from the cathode to the negative glow is to produce the supply of electrons from the neighbourhood of the cathode necessary to keep the discharge going. the dimensions of this part of the discharge are independent of the distance between the cathode and anode; at very low pressures this part may occupy a length of several centimetres, but at atmospheric pressure they are crowded into a very small fraction of a milli- metre and as far as length goes occupy a negligible portion of the sparks at such pressures. the crookes’ dark space, though it appears dark in contrast to the negative glow, is not devoid of luminosity; indeed seeliger, who has made a spectroscopic examination of the dark space, finds that there are some lines, such as the balmer series lines, which are almost as bright in the dark space as in the negative glow. but many lines are much stronger in the negative glow than in the dark space. beyond the negative glow there is another comparatively dark region called the ‘‘ faraday dark space’; the length of this is very variable even when the pressure is constant, as it 1s sensitive to any change in current. beyond this and reaching right up to the anode is a juminous column called the positive column. the luminosity in some cases is fairly uniform in intensity, but when the pressure and current are between certain limits this column may exhibit remarkable alternations of dark and bright spaces called striations, such as are shown in fig. 14. under some cir- cumstances a dark space round the anode has been detected by several observers. | when the distance between the electrodes is considerable and the pressure not very low, the positive column forms by far the greater part of the discharge; thus at atmospheric pressures all but a fraction of a millimetre of the discharge next the cathode will consist of the positive column. distribution of the electric force along the discharge -—the electric force is very large indeed in the part of the dark space next the cathode, but diminishes rapidly towards the negative glow. in the negative glow itself it is smaller than in any other part of the discharge; passing the negative glow, the electric force increases in the faraday dark space, until the positive col- umn is reached. when the positive column is of uniform luminos- ity the electric force in the column is constant until quite close to the anode, when there is an abrupt change of potential of about 20 volts, called the anode fall of potential. when the positive column is striated, the alternations of luminosity in the positive column are accompanied by alternations in the intensity of the electric force, the maxima of the electric force occurring gases, electrical properties of at the bright parts of the striae, the minima at the dark. from _ ax ; the equation ge 47s where x is the electric force in the direc- tion of x and p the density of the electrification, we see that there is an excess of positive electricity in the cathode dark space and of negative in the faraday dark space; in a uniform positive column there is no appreciable excess of electricity of one sign over that of the opposite, while in the striated positive column there is an excess of negative electricity on the cathode side of a bright part of a striation and of positive on the anode side. cathode fail of potential —until the glow next the cathode covers the whole of the electrode the difference of potential be- tween the cathode and the negative glow is constant, depending on the gas and the material of which the cathode is made, but being independent of the pressure of the gas and the strength of the current. this constant difference of potential is called the “ cathode fall of potential,” and there is evidence to show that it is equal to the minimum potential that can produce a spark through the gas. its value, for different gases and different electrodes is given in the following table, due to giinther- schulze:— table vitt. -2-72 4 -2.92 ~o-43 --12 es oe further light is thrown on the origin of ionisation in gases by the study of the distribution of electric force in the neighbourhood of the cathode. aston has shown that at low pressures the force in the neighbourhood of the cathode is a linear function of the distance from the cathode, so that if v be the electrostatic point at a point distant x from the cathode as c{d—x) ao (22) when c and d are constants. if p is the density of the electrifica- tion at the point x: 4zp=d?v/dx*, hence for this law of force pisconstant. we have to consider what law of ionisation is com, patible with this result. it can be shown that to make p constant the ionisation must be independent of the distance from the cathode, a result which seems much more probable if the ionisa- tion is due to radiation than if it were mainly due to ionisation by collision. we see from equation (22) that dv/dx vanishes when x=d. this is the place where the negative glow begins. the potential difference between the cathode and the boundary of the negative glow is called the cathode fall of potential and is a quantity of great importance, of which many measurements have been made which are givenin table viii. the cathode fall of potential depends on the nature of the gas and of the electrodes but is independent of the pressure. the consequences of the hypothesis of ionisation by radiation are in fair agreement with the results of experiments; they are as follows :— | _ the normal cathode fall of potential is independent of the pressure of the gas. the thickness, d, of the dark space is inversely proportional to the pressure. the cathode fall of potential v is connected with i the density of the current carried by the positive ions by the relation:— = my\\? v= (my ere 2€ where m is the mass of a molecule of the gas. the cathode fall of potential is on this view equal to the smallest potential difference that can produce a spark through the gas and the thickness of the dark space at any pressure is the length of the spark which at that pressure passes with the minimum potential difference. (23) 153 we see from cquation (23) that in order to produce the dis- charge a definite current density as well as a definite potential difference is required. this density, for a flat electrode, is as we see from equation (23) inversely proportional to d? and therefore directly proportional to the square of the pressure. the values of v and i at a pressure of 1 mm. of ig for plane cathodes (1) of aluminium, (2) of iron, for different gases have been determined by giinther-schulze (zeit. fiir physik, 20.1). table tx. aluminium v. volts v. volts i(am/cm?) t(am/cm*) ile 1-07 153 1-187 161 ne. 1:81 145 2-000 153 ar 14-07 150 15°5 166 n 38-4 215 42:3 256 h : 9:0 ig? 9:96 250 qo 54°7 250 60-6 326 v depends on the nature of the electrode as well as upon the gas. giinther-schulze’s experiments indicate that v can be expressed approximately in the form:— v= 35‘5e+e where a is a quantity depending on the gas and e, a quantity called the electrochemical normal equivalent, which is given in the last row of table viii. the values of a in volts for different gases are he, 175; ne, 1609; ar, 174; n, 265; h, 290; o, 350. the preceding values refer to what are called the normal values when the current flowing through the tube is not large enough to cover the whole cathode with glow, so that an increase in the current may occur through an increase in the area over which the current is spread and not from an increase in the mean density of the current. giinther-schulze gives the following empirical expression for the dark space, d pd = (constant) avi when p is the pressure, \\ the free path of the molecule at atmos- pheric pressure, and v; the ionising potential of the gas. when the current is so great that the whole electrode is covered with glow v, i and d are no longer constant; the cathode potential fall increases and the dark space diminishes as the current i increases. according to aston’s experiments (proc. roy. soc., 86, p. 178), the following relations exist between these quantities :-— a br aga a p yl fvi v = e+—— p here » is the pressure and a, b, e, f constants. as the current increases the values of v may increase to many thousand volts and thus be hundreds of times the normal value. according to aston there is no sudden jump in potential at the cathode itself, some observers (e.g., westphal) have thought that such a drop occurred and that it amounted to a considerable fraction of the whole potential fall. | though in many gases and especially in oxygen there is a very sharply defined boundary to the dark space, it must not be thought that no radiation comes from the dark space; it is only for certain types of radiation that the radiation from the dark space is absent. thus for the type of radiation which is instru- mental in producing ionisation, there is no discontinuity in the radiation as we pass across the boundary of the dark space, and seeliger and his co-workers have shown that even for visible radiation the change in intensity on crossing the boundary depends to a great extent on the wave-length of the light. the electrical conditions inside the dark space differ very much from those outside; inside we have a great preponderance of positive ions over electrons, while outside the two are equal; again outside the density both of free electrons and of positive ions is much greater than inside, and there is recombination of electrons and positive ions outside and not inside. it is not surprising 154 therefore that radiation produced outside the dark space should differ substantially from that produced inside. anode fall ef potential —unlike the cathode fall of potential which is spread out through a length equal to the thickness of the dark space, the anode fall as skinner has shown occurs quite abruptly so far as it can be tested by experiment. if this fall took place in molecular distances the electric attraction might be sufficient to drag positive ions out of the electrode itself. by using a cathode heated to incandescence, and therefore emitting a plentiful supply of electrons, we can reduce the cathode fall of potential to a small fraction of its normal value; we cannot, however, with a luminous discharge get rid of the anode fall; thus in the arc discharge the anode fall of potential is greater than the cathode fall. matthies has shown that, in chlorine, bromine and iodine, the anode fall of potential may rise to hundreds of volts, that in air or hydrogen being only about 18 volts. reichenheim and gehrke utilised this fact to get positive ions of sodium and potassium projected with great velocity. they made the anode of a mixture of the halogen salts of these metals and graphite, and worked at a very low pressure; under the action of the discharge the halogens were liberated from the anode, and the large anode fall they produced was sufficient to project sodium and potassium ions from the anode with great velocity; this stream of positive ions constitutes what is known as “ anode rays.” the electric force in the positive column is a linear function of the pressure; it depends slightly on the diameter of the tube through which the discharge is passing; it also depends on the current through the tube; in most cases, though not invariably, an increase of current produces a decrease in the electric force. the condition determining the electric force in the positive column is that it should give to an electron during its free path the amount of energy that will enable the electrons to produce by collisions as many ions per second as are lost during the same time by recombination. striated discharge—the form of discharge when the positive column is striated is so beautiful and remarkable that it has attracted a great deal of attention. to get this type of discharge the current and pressure must be within certain limits. the striations are developed more readily in mixtures of gases than in a pure gas; in fact some physicists have advanced the view that they could not be obtained in an absolutely pure gas. there is no doubt, however, about their occurrence in gases in which great attention has been paid to purification. norbeck could not get them in pure nitrogen or pure helium, though they were conspicuous as soon as a trace of impurity was admitted. ni- trogen and helium are gases in which, when pure, the carrier of negative electricity is always an electron; in these gases the electron does not join on to a molecule and become a negative ion. spottiswoode found that, in some cases when the positive column showed no signs of striation when observed in the usual way, striations moving rapidly down the tube could be seen when the discharge was observed after reflection in a rapidly rotating mirror. aston and kikuchi, who have studied this effect in neon and helium, are of opinion that the striations are moving in these gases with the velocity of sound; it must be remem- bered, however, that the velocity of sound in many gases is of the same order as the velocity of a positive ion under the electric forces in the positive column, so that this result does not neces- sarily prove that the moving striations are analogous to sound waves. the distance between the striations increases as the pressure diminishes (in hydrogen the distance is inversely proportional to the square root of the pressure); it depends upon the size of the tube: the striations are nearer together in narrow tubes than they are in wide. the distance between the striations also depends upon the current. when several gases are in the tube, spectro- scopic observation of the bright parts of the different striations shows that we may have one set of striations corresponding to one gas, another to another, andso on. thus crookes observed in a tube containing hydrogen three sets of striations, one set red, another blue and a third grey; the spectroscope showed that the first was due to hydrogen, the second to mercury vapour and the gases, electrical properties of third to hydrocarbons. the striations are often curved with their concavities turned to the anode. to get a general idea of the causes which might give rise to stratifications, let us consider a case where the current is carried entirely by electrons, the positive ions being regarded as im- movable in comparison with the electrons. let us consider a stream of electrons emerging from the negative glow; these electrons will have very little energy and will be unable to jonise the gas. the electrification beyond the negative glow on the anode side will be due to electrons and will be negative so that the electric force will increase towards the anode. as the electric force increases the energy of the electrons also increases, until it is sufficient to enable the electron to ionise the gas and produce positive ions and electrons; the increase in the number of ions will check the rate of increase in the electric force. the connec- tion between the ionisation and this rate of increase is in the case we are considering represented by a very simple equation. tor if 7 and m represent respectively the number of negative and positive ions per unit volume, x the electric force, and x the distance from the cathode i 4m(n—mye (24) if the current tis carried, as we have supposed, by the electrons, ncu = t, where 4 is the velocity of the electron. if we neglect the current carried by the positive ions, then when things have reached a steady state the number of positive ions produced in any region per second must equal the number which disappear owing to recombination. hence, if g is the rate of ionisation, a the coefficient of recombination, g=amn or m=g/an; and (24) is equivalent to dx 4m 4mgeu = (25) thus as long as g vanishes, «[x/dx is positive, but as soon as ¢ becomes finite the rate of increase of x will be retarded; as x increases g increases, and when e*¢u?= a2, (lx/dx will vanish; but though x reaches its greatest value at this point, the vafues of « and g, which depend on the energy acquired by the electron, will continue to increase beyond it. for the energy acquired by dx cit au an electron depends on f{ xar, taken over a distance measured by the free path of the electron; at low pressures this may be a centimetre or more, and the place where | xdx is a maximum will be beyond that where x is a maximum by a length of this order. thus after x has reached its maximum w and gq will in- crease and dx/dx will become negative, so that x will diminish; the diminution in x will ultimately produce a diminution in xd and also in # and q; the rate of decrease will slow down; x will attain a minimum, and begin to increase again when similar changes will be repeated. thus the curve which represents the relation between x and x will resemble fig. 35, where the cathode is on the x left, giving alternate maxima and minima for the value of x; and { xar, theenergy acquired by an elec- tron, will vary periodically along the path of the discharge. there are two values of this energy which are of special importance in connection with discharge through gases, one the ionising potential we have already referred to, the other, sometimes called the ‘“‘ radiation potential,” is the energy which the electron must pos- sess to make the gas luminous. the radiation potential is less than the ionising potential, and electrons with energy between these potentials will make the gas luminous but will not ionise it. thus the molecules of the gas will give out light but will not be charged. when the energy of the gas exceeds the ionising po- tential the luminous molecules are or have been charged. if the oe ag x fig. 15 cathode gases, electrical properties of variations in the energy along the line of discharge are large enough to make it sink below the radiation potential, then along the discharge we shall have: (1) places where the energy is be- low the radiation potential,—these will be dark; (2) places where the potential is between the radiation potential and the ionising potential,—the molecules here will be luminous and uncharged and will, therefore, not move under the electric field; (3) places where the molecules are luminous and charged, = these molecules will move down the tube towards the cathode with the velocity which the positive ion acquires under the electric field. this velocity, when the pressure is low and the field several volts a centimetre, as it is in the positive column, may be many thousand centimetres per second. place (1) corresponds to the dark parts of the striations, (2) to the stationary luminous parts, while (3) is the origin of the striations moving down the tube observed by wulner, spottiswoode, aston and kikuchi. cathede rays and the discovery of the electron.—in 1859 pliicker observed on the glass of a highly exhausted tube in the neighbourhood of the cathode a bright greenish yellow phosphor- escence, which changed its position when a magnet was brought near to it. about ro years afterwards hittorf showed that a solid body placed between a pointed electrode and the walls of the tube cast a well-defined shadow of such a shape as to show that the agent producing the phosphorescence travels in straight lines at right angles to the surface of the cathode. the name “cathode rays ” for the cause of the phosphorescence was in- troduced by goldstein, who made many important investiga- tions on their properties. the opinion held by goldstein and generally in germany was that cathode rays were waves in the ether. varley and crookes advanced the view that they were electrified molecules shot off at right angles to the cathode. the discovery by hertz that the cathode rays could pass through thin layers of gold leaf was difficult to reconcile with this view. the evidence in favour of the cathode rays being electrified par- ticles was increased by perrin’s discovery that when a pencil of the rays entered the opening in a faraday cylinder they gave to it a charge of negative electricity. one difficulty which had been urged against the rays being negatively electrified, viz., that, though they were deflected by a magnetic force, an electric force produced no effect upon their path—was removed by j. j. thomson, who showed that the absence of deflection was due to the gas in the tube acting as a screen and protecting the particles from the electric force. as the gas in the vacuum tube is a conductor of electricity the rays move inside a conductor of electricity, and so will not be affected by an external electrified body. thomson showed that when the vacuum was very high, so that there was but little gas in the tube, the cathode rays were deflected by an electric and magnetic field, and that the direction of the deflection indicated a negative charge on the particles. his measurement of the deflection by known electric and magnetic forces led to a determination of the mass of the particles which carried the charge, and showed that these par- ticles were not atoms or molecules but something with a mass not one-thousandth part of the mass of the lightest atom known, that of hydrogen. the deflection due to electric and magnetic forces can be cal- culated as follows. suppose that the particles are travelling horizontally between two parallel horizontal metal plates a, b, maintained at a constant difference of potential, there wiil be a vertical electric force f acting between the plates, and if the axis of y is vertical the equation of motion of the electrified particle when it is between the plates is d?y = df d if y and 7 are both zero when the particle enters the region between the plates, then, when it leaves this region, after a time dy _ fe dt nt since the electric force is at right angles to the direction of motion of the particles, v the velocity of the particles will not alter, and fe 5 eae oar m 155 if the deflection is small, ¢=//» where /is the length of the plates. thus y fel mv suppose the particles strike a photographic plate or a screen covered with a phosphorescent substance at a distance l from the end of the plates, the y displacement at this place produced by the electric force, is given by the expression paces fell =< (-+ +l) ? ie anv? mv\" magnetic deflection of the rays.—if the rays go through a uniform magnetic field of length / and strength h, then if the magnetic force is vertical the force acting on the moving par- ticles will be ilev, and will be at right angles to the magnetic force and also to the direction of motion of the rays; 7.e., it will be at right angles to the plane of the paper; if z is the displace- ment of the particle in this direction y= = hev. from this we see that the value of z at the screen is given by _ he “, my met hence — ae sens (26) my f we(l) zf and - 7 (27) thus the measurements of y and z, the electric and magnetic deflections, give the values of e/m and 2. the expressions for y and z have been obtained on the sup- position that the electric and magnetic fields acted one at a time and not simultaneously. if, however, y and z are small, their values will not to a first approximation be altered if the electric and magnetic deflections occur simultaneously. thus by making the cathode rays pass through superposed electric and magnetic fields, e/s and t can be got with one exposure by measuring y and z on the screen or photographic plate. since from equation (26) above 2*/y is constant as long as e/m is constant, we see that all the particles of the same kind, whatever their velocity, would strike the screen or plate on a parabola, and that if the rays were a mixture of particles of different kinds each kind of particles would trace out a different parabola. since z/y only depends upon 2, all the particles moving with the same velocity will strike the screen in a straight line. the determination of e/m for the cathode rays led to results of fundamental and far-reaching importance, for it was found that all the cathode rays had the same value for e/m, and that moreover while for a charged atom of hydrogen in liquid elec- trolytes e/m was equal to 104, when e was measured in electro- magnetic units, the value of e/m for the particles in the cathode rays was considerably more than one thousand times this value. thus if e were the same for the particle as for the hydrogen atom (and we shall see later that this is the case) the mass of the cathode particle is only 7/94 of that of an atom of hydrogen, the smallest mass which hitherto had been recognised. again it was found that whatever metal might be used for the cathode, or whatever might be the gas in the discharge tube, the value of e/m was unaltered. as those particles must have come either from the electrode or the gas, it follows that the particles of the cathode rays are a constituent of the atoms of all the chemical elements. these particles are called ‘ electrons.” after the electrons had once been detected in the cathode rays they were very soon detected under many other conditions and found to be of very widespread occurrence. thus, for example, it was found that streams of electrons are given out by in- candescent metals, the rate of emission increasing very rapidly with the temperature. this has received a very important in- dustrial application in what are known as “ hot wire valves,” 156 at which a current from a hot cathode passes through a vessel in which the vacuum is so high that the gas takes 1.. part in the discharge; the current, in some cases amounting to several amperes, is carried entirely by electrons. lenard found that they were emitted by metals exposed to ultra-violet light. they are emitted when r6ntgen rays strike against matter and by radio-active substances. the speed of the electrons ejected cither by ultra-violet light or by rentgen rays does not depend upon the intensity of the radiation but only upon the wave length. the energy acquired by the electrons is=/yv, where v is the frequency of the radiation and # planck’s constant. since the cathode rays are deflected by electric and magnetic forces proportionally to the magnitude of these forces, we can use the deflection of the rays as a measure for electric and magnetic forces. as these rays have practically no inertia they are especially adapted to measure very rapidly alternating forces which could not be detected by any index having an ap- preciable mass. the cathode ray oscillograph, an instrument by which electric and magnetic forces are measured by the deflection of cathode rays, has already been used in many investigations, and is a very important aid to research. another property of cathode rays is that when they strike against matter they gen- erate r6ntgen rays, the hardness of the latter increasing with the speed of the former. the mass of an electron depends upon its velocity; this effect is not appreciable unless the velocity is comparable with that of light but the increase in mass becomes very marked when as in the case of the 8 rays from radio-active subjects the velocity exceeds 1o!®cm./seconds. on the theory of relativity the relation between m, the mass of an electron mov- ing with the velocity v and mm» the mass of an electron at rest is pilg vi—v/e where c is the velocity of light. this relation seems to be in agreement with the measurements which have been made on the masses of the @ particles. if the electron is regarded as a small sphere of radius @ then it i, = 2¢e =~. ace where ¢ is the charge on an electron in electrostatic measure. the value of e/mo found by experiment is 1-78 x 107 xc, the value of ¢ is 4-8 x10-!9, and c=3 x1o!®, hence the value of a from the preceding equation is equal to 1-9 x107!8 cm. positive rays-—goldstein discovered in 1886 that, if the cathode in a highly exhausted tube was perforated, bundles of a luminous discharge streamed through the aperture into the space behind the cathode. the colour of this discharge depends upon the gas in the tubc; thus in hydrogen it is rose colour; in air, yellowish. the colour of the light due to these rays is not the same as that produced when cathode rays pass through the gas. in some gases the difference is very striking; thus in neon the light due to the cathode rays is pale blue, while the discharge which streams through the cathode is a gorgcous red. goldstein called the rays which stream through the hole in the cathode kanalstrahten; but as they have been proved to consist of posi- tively charged particles it seems more natural to call them ‘positive rays.” these rays produce phosphorescence when they strike against glass and many other substances, though the phosphorescence is generally of a different colour from that produced by cathode rays. they also affect a photographic plate. it was at first thought that the positive rays were not deflected by a magnet, as magnetic forces which produced large deflections of cathode rays had no appreciable effect upon positive ones. wien showed, however, by using very strong magnetic fields, that they could be deilected and that the direction of the deilection indicated that they carried a charge of positive elec- tricity; they can also be deflected by electric forces. by measuring the detlection provided by electric and magnetic fields we can determine by using equations (26) and (27) the value of e/m for the particles which constitute the rays. the result is of great interest. instead of, asin the cathode rays, c/m having the same follows from the laws of electromagnetic action that mo = gases, electrical properties of value for all the carriers, we find that e/m has many different values separated by finite intervals; and instead of e/m being equal to 1:78 x10\", as in the cathode rays, we find the greatest value of e/nz is 104, which is the same as its value for a charged hydrogen atom. ‘the values found for e/# depend on the gases in the discharge tube; the outstanding result is that all these values of m correspond to masses of atoms or molecules of the chemical elements or compounds. thus while the determination of e/m for the cathode rays shows that in a gas at a very low pressure the carriers of the negative electricity are all of the one type, being electrons whose mass is excceclingly small compared with that of any atom, the determination of e/m for the positive rays shows that the carriers of the positive electricity are of many different types; and that all these types correspond to atoms or molecules of the chemical elements or compounds. it has already been shown that if charged particles, after passing through electric and magnetic fields, are received on a screen or photo- graphic plate, all particles, for which e/ is the same, strike the plate on a parabola, and that for each different value of e/m there is a separate parabola. these parabolas are shown in fig. 16, which is a reproduction of a photograph made by allowing the positive rays in a tube con- taining gases liberated by heating a certain mineral to strike against a photographic plate; taken from the top downwards they correspond respectively to the atom of hydrogen, the molecule of hydrogen, the atom of helium, the atom of carbon with two charges, the atom of nitrogen with two charges, the atom of oxygen with two charges, the atom of carbon with one charge, the atom of nitrogen, the atom of oxygen, the molecule of water, the molecule of coand that of ne (these form one parab- ola}, the molecule of oxygen, the molecule of co, and the atom of mercury. we find that many of the atoms can carry more than one charge, for when we find a parabola corresponding to one value of e/m we frequently find another corresponding to twice this value; thus carbon, nitrogen and oxygen occur very fre- quently with two charges, other atoms such as argon with two and three charges, while mercury atoms have been detected with i, 2, 3, 4, 5, 6, 7 charges. it is significant that the atom of hy- drogen never occurs with more than one charge. multiple charges generally occur on atoms but not on molecules; there are, however, some molecules such as co on which double charges have been found. some of the positive particles, after passing through the hole in the cathode, lose their positive charge and become uncharged, and some of these neutral particles acquire a negative charge; thus mixed with the positively clectrified particles there are some negatively electrified ones. this power of acquiring a negative charge is confined to certain atoms; thus while the atoms of hydrogen, carbon, oxygen, fluorine occur with a negative charge, the atoms of nitrogen, helium, argon and neon do not. it is exceptional for a molecule to acquire a negative charge—the molecules of oxygen and carbon, however, can do so. gases, electrical properties of the er,uation of a parabola formed by a particle on the photo- graphic plate has already been given er m where z is measured parallel to the displacement due to the raagnetic field and y to that due to the electrostatic. c is a quantity which depends on the strength of the electric and magnetic fields and on the position of the photographic plate. if, asin fig. 17, we draw a line parallel to the axis of z, the inter- cept made by a parabola on this line will be proportional to (e/m)?; thus, if the top parabola is due to the atom of hydrogen, the next to the molecule of hydrogen, the third to the atom of helium and the fourth to that of oxygen, the intercepts aii, ah, att,, ao are in the proportion of 1, 1/¥ 2, 1/4. thus by com- paring the intercept made by any parabola x with that made by a the parabola due to the hydrogen atom we can find the molecular weight of the substance producing the parabola x. positive rays as a method of chemical analysis——since from the measurement of the positive ray photographs we can deter- mine the molecular weight of the gases in the discharge tube, we can analyse a gas by putting a small quantity of it in a discharge tube and taking a photograph of the positive rays. it is thus a method of chemical analysis, and its application has already led to the detection of several new substances. in fact, though it has only recently been introduced, more substances have been discovered by this method than have ever been discovered by spectrum analysis. the method has many advantages. in the first place only a very minute quantity of the gas is required; a small fraction of a cubic centimetre of gas at atmospheric pres- sure is all that is required to fill the discharge tube at the pressure at which the positive rays are produced. again, the method is very sensitive, as it will detect the presence of a gas which only forms a small percentage of the gas in the tube. the method not only detects the presence of the gas, but at the same time determines its molecular weight. it indicates, if the gas 1s an element, whether it is monatomic or diatomic; for if it is diatomic it will give rise to two parabolas, one due to the atom, the other to the molecule. the absence of double or negative charges will suggest that it is a compound and not an elementary gas. the only ambiguity is that it does not distinguish between two sub- stances of the same molecular weight; thus co:, and n.o give the same parabolas, as also do co and no»: we can often, however, remove this ambiguity by putting substances in the tube which would absorb one gas and not the other, and testing whether or not this has removed the parabola. use of positive rays to determine atomic weight.—the meas- urement of the parabolus give, as we have seen, the atomic weight of the elements producing them; they can therefore be used to determine the atomic weight of elements which can be introduced in a gaseous state into the discharge tube. this “method has the great advantage that the presence of impurities does not affect the result. mr. aston has lately, by the use of a positive-ray method for determining atomic weights (sce atomic. 157 weticits), found the very important fact that, if oxygen is taken as 16, the atomic weights of the elements with the exception of hydrogen are represented by whole numbers. thus in working with chlorine he found no substance with an atomic weight of 35:4, but two substances with atomic weight of 35 and 37 re- spectively; he regards these substances as identical in chemical properties and inseparable by chemical reactions, and ordinary chlorine as a mixture of about 3 parts of (357) and one part of (37°). mr. aston, by the method of positive-ray analysis, has discovered and measured the masses of the isotopes (see iso- topes) of most of the chemical elements. the charges of electricity carrted by gaseous ions and elec- trons-——the defection of cathode and positive rays by electric and magnetic forces supplies a method for finding the value of e'm; for the determination of ¢, the charge of an ton, other methods have to be employed. one such method used by j. j. thomson is based on the important investigation of c.t.r. wilson on the effect of ions on the deposition of clouds and fogs from supersaturated air. if dust-free air saturated with water vapour is suddenly cooled by expansion, no cloud or fog is de- posited unless the supersaturation due to the cooling is very large. c. t. r. wilson found that if ions are present in the gas they act as nuclei round which drops of water are deposited with a super- saturation much below that required for gas free from ions. a beautiful application of this is the detection of the path of ana particle fromaradioactive substance. thea particle produces by collision ions all along its path; 1f the damp gas through which the particle is passing is suddenly cooled by expansion, drops of water will deposit on the ionsandthus mark out the path of the particle. one of mr. wilson’s photographs of such a path is shown in fig. 18. afr. wilson found that less supersaturation is required to deposit water on negative than on positive ions. a » a 1% ‘ % : ty tl ’ 4 this result can be applied to find the number of ions in a moist gas, forif the gas is sudidlenly expanded by an amount sufhcient to deposit drops on ions, but not sufficient to produce condensa- tion in their absence, then each ion may be made the centre of a drop, and the problem of counting the ions is reduced to that of counting the drops. we can calculate the amount of water that will be deposited by any given expansion of the air: hence since we know the volume of the water we can determine the number of drops if we know the volume of a single drop. observation of the rate at which a drop falls under gravity will give the size of the drop, for stokes long ago showed that the velocity of a rain drop falling aia ted 2 a\" under gravity is given by the equation v= — g—: when v isthe velocity of the drop, a its radius, y the viscosity of the gas, g the acceleration due to gravity, and p the density of the gas. 1t has been found that, with the exceedingly fine drops formed round ions where the radius of the drop is comparable with the free path of the molecules of the gas, the velocity is (: +5) times a 158 that given by the above equation when c isa constant, and p the pressure. but though this correction makes the relation between a and v a little more complicated, it still enables us to determine a when v is known. thus the radius, and therefore the volume, of the drop can be determined, and from this, as we have seen, we can deduce the number of ions. let # be this number per unit volume; then if a current of electricity is sent through the gas by an electric force x, the cur- rent passing through unit area will be xceu when u is the mean velocity of the positive and negative ions under the force x. we know that it is proportional to the force and for a force of one volt per cm. is 1-5 cm./second; and hence when x is known u is known, the current zeu can be measured, and hence ne deduced; as 2 has been found by the drops, the value of e can be determined immediately. this was the method used by j. j. thomson; a simpler method used afterwards by h. a. wilson was to get drops round the negative ions alone by using an ex- pansion that would deposit moisture on negative but not on positive ions. he then showed the rate of fall of these drops, first under gravity alone, and then under a vertical electrical force x, acting on the drop in the same direction as gravity. thus, when the electric field is acting, the force on the drop is xe+4rpaig, and when it is off the force is only $mpa*g. thus, if vi, v are respectively the velocities of the drop when the field is on and off, xe+smparg — 4mpa’g v or xe = $rp0ig— from v, the rate of fall when the field is off, we can calculate as before the radius of the drop, and from the preceding equation we can determinee. millikan(pail. mfag., 34, p.1), who has made most extensive and accurate investigations on the value of ¢, usec a modification of the preceding method. instead of producing water drops by expansion on the ions, he obtained, by means of a sprayer, minute drops of oil; he observed the motion of onc of these under an electric field in a gas which was subject to some ionising agent, and from time to time an ion would strike against the drop and alter the charge; this would alter the velocity, and from the alteration of the velocity he could by a formula similar to that just given calculate the charge communicated to the drop by the ion. the value obtained for e by this method is, in elec- trostatic units, e€ = 4:77x107% if m is the mass of an atom of hydrogen the value of e/m can be determined by the positive-ray method, so that as e is known m can be determined. when m is known avogadro's constant and the number of molecules of hydrogen in a cubic centimetre of gas at standard temperature and pressure are at once determined. thus the study of the electrical property of gases gives the most accurate values available of two of the most important con- stants connected with the constitution of matter. by studying electrified atoms and molecules, we are able to determine their masses and their properties with an accuracy far beyond that attainable by any method which can be used when they are in the normal state. see sir j. j. thomson, conduction of electricity through gases (1926) ch.) gasolene, commonly known in great britain as petrol, is a motor fuel particularly suitable for use in motor vehicles. it has been officially defined by the american society for testing materials as a refined petroleum naphtha which, by its compost- tion, is suitable for use as a carburant in internal-combustion engines. petroleum naphtha, as understood in the above defini- tion, is any product of petroleum of which not less than 16:75 distils below 347°f. (175°c.) and not less than 95% below 464° f. (240°c.) when subjected to distillation according to the current tests of the american society for testing materials, —-. gasolene the term gasolene (natural gasolene) is applied also to a liquid product of natural gas, which, however, in order to be market- able, must be blended with light grades of petroleum gasolene. on the other hand, among other motor fuels derived from petro- lecum, which, even in its crude form, 1s sometimes used for this purpose, are kerosene, which is suitable for tractors, and the distillate fuel oil employed in the semi-diesel type of engine. besides serving as fuel in motor-vchicle engines, gasolene is used for aeroplanes, motor-boats, motor rail cars and many kinds of stationary engines. the increase in the use of motor vehicles, particularly in the united states, has made the question of gasolene supply a mat- ter of vital concern. the consumption in the united states dur- ing 1924 was approximately 7,750,000,000 gal., nearly 500 gal. for each of the 16,000,000 cars then in service. the largest con- suming countries outside of the united states are, in order: great britain, france, canada, argentina, belgium, australia and italy. the european consumption for 1924 is estimated at i,300,000,000 u.s. gal. (£ of the imperial gallon). the follow- ing tabulation, based on figures supplied by the u.s. dept. of commerce, shows the production and consumption in the impor- tant countries in typical years. production and consumption ef gasolene country : ; ie cae anal veit production consumption (in u.s. gal.) united states ls. gal. gross per capita i9iig i 460,038,200 1,255,218,538 1923 7:555,945,143 | 6,685,035,280 ae 1924 8,959,690,220 7,780,625,085 69°3 great britain imperial gal. i9il4.. ; 5,140,364 141,029,305 1923 71,000,000 417,027,593 ies 1924 95,000,000 549,600,000 11-6 france metric tons 1913 ao 63,113,000 1923 3,848 261,011,000 1924 4,000 297,234,000 canada imperial gal, 1924 170,000,000 260,252,00c 29:0 argentina liters 1924 50,900,000 68,200,000 8-0 belgium 1923 35,155,185 ns 1924 57:272,142 7°6 australia 1922-3 metric tons 46,800,000 8-6 italy 1913 2,800 12,046,900 i 1923 898 48,226,700 i-3 germany 1913 1922 1923 88,483,000 70,214,000 43,794,000 rumania 1913 1922 1923 10,889,000 27,089,000 31,655,000 422,019 285,097 300,847 poods 12,748,800 russia 1923-4 14,750,000 gasolene supply.—the supply of gasolene depends on the available petroleum reserve (see petroleum 21.316), efliciency in refining methods, and economic utilisation. the increase in the yield of gasolene from a given amount of petroleum is to be expected chiefly from more careful fractionation, improved cracking methods, and raising the upper distillation limits in the production of straight run gasolene by straight distillation, ac- cording to an estimate made by the united states bureau of mines, the gasolene produced from crude oil amounts to gasquet—gdynia about 33% of the oil entering the refineries. this percentage might be increased to 55, in the opinion of the committee of eleven on conservation, american petroleum institute. of greater promise are possible improvements in engine design, as a result of which the energy available from a given amount of gasolene may be greatly increased (sce internal combustion engine). it is estimated by c. f. kettering, president of the general motors research corporation, that the percentage of energy in gasolene transformed into work is, on the average, only 5, but that this might be increased to ro through structural mechanical changes, thereby almost doubling the transportation efficiency of a gallon of gasolene. afarketing methods.—illighly specialised methods for the mar- keting of gasolene have been developed, particularly in the united states. the gasolene is shipped from refineries in tank cars or ships (tankers) to large storage terminals and thence to smaller storage tanks along railways. from these it is taken by motor tank truck to “‘ service stations,” garages and curb pumps, where it is pumped inte motor vehicles. it is estimated that there are about 100,000 “ service stations ” in the united states, while innumerable curb pumps are to be found on the city streets and along country highways. similar marketing methods are employed in canada. in great britain and france the dis- tribution of petrol was largely effected by tins but the gasolene pump has come into wide use. public service stations for auto- mobiles are now to be seen also in the principal cities of germany, belgium, northern italy and other european countries. anti-knock fuels.—a significant recent development has been the production of a gasolene designed to eliminate or reduce the “knock ” of an engine. there are also substances on the market advertised to effect the same result if added to ordinary gaso- lenes. one such compound, known as ethyl gasolene, consists of commercial gasolene to which has been added a small fraction of 1% of a synthetic oil containing tetraethyl lead. anti-knock fuels are said to increase the mileage per gallon, to reduce carbon deposits in the motor, to facilitate starting and to improve en- gine efficiency in cold weather. of greater importance, however, is the fact that the elimination of ‘‘ knock ”’ removes the barrier to the introduction of an automobile engine which would greatly increase the mileage per gallon and otherwise compare favoura- bly with the performance of engines now in use. gasolene substitutes —benzol, derived as a by-product from coking coal, is used to a limited extent as a blend with gasolene in the united states, the limitation being duc to the relatively small production of benzol. the substitutes for gasolene which promise the greatest usefulness are oil derived from shale and especially liquid fuel derived from coal. the use of alcohol (¢.v.) as a fuel for internal combustion engines is growing in great britain, where extraordinary measures have been taken to en- courage research and production of this fuel. the french govt. also has undertaken research to this end and has encouraged the development of a local alcohol industry, the product of which can be used as fuel. the use of alcohol as a motor fuel in germany was tried some years ago; more recently benzol, prepared by the carbonisation of coal and lignite, has been used as a motor fuel. methanol, also, produced on a commercial scale from carbon monoxide and hydrogen by the so-called bergius method for the liquefaction of coal, has attained importance as a substitute for gasolene. (see coal; fuel problems.) bibliography.—a merican petroleum supply and demand (1925); u.s. department of commerce, world trade in gasolene (1925); t. a. boyd, gasolene, what everyone should know a bout it (1925); kk. g. mackenzie, petroleum motor fuel; a.c, fieldnerand r. l. brown, complete distillation of coul and motor fuel; m. c. whitaker, alcohol motor fuels, (l. m. f.) gasquet, francis aidan (1846- ), english roman catholic divine and historian, was born in london oct. 5 1846. he was educated at downside college, bath, afterward becoming superior of the downside benedictine monastery (1878-84). created cardinal in 1914, in 1918 he was appointed prefect of the vatican archives. he has produced various works on mediae- val church history and liturgies, among them being henry vill. i59 and the english monasteries (1888-9); a short history of the catholic church in england (1903); parish life in mediaeval england (1906); the bosworth psalter (1908); monastic life in the afiddle ages (1922); his holiness pope pius xi. (1922). gaul, gilbert william (1855-1919), american painter (see 11.532), died in new york dec. 21 1919. hewas awarded a gold medal at the appalachian exposition, knoxville, in roto. gauvain, auguste (1861- ), french journalist and diplomat, was born at vesoul oct. 6 1861. from 1889 to 1892 he was on the staff of le journal des debats and in 1893 became general secretary to the european commission of the danube. in 1904 he was appointed french secretary to the central office of international transport at berne. in 1908 he returned to the staff of le journal des debats and from that time directed its foreign policy. by the continuity and clearness of his views, his freedom from bias and, above all, by his exceptional talent as a writer, m. gauvain came to exercise considerable influence. tle gave early warning of the menace of german imperialism, and more especially of the danger threatened to europe by the con- dition and policy of austria-hungary. he was an unceasing advocate of energetic action and moderation in aim on the part of the allies in the world war. gauvain, who became a member of the academy of moral and political sciences, pub- lished verbatim in 14 volumes all his articles in le journal des debats from 1908 to 1920. (see france: hisrory) among his other works are: les origines de la guerre europeenne (1915); l’ europe avant la guerre (1917); l’affaire grecque (1917); la question yougoslave (1918); l’encerclement de l’ allemagne (1919). gdynia or gpincen.—this polish seaport and naval base is situated on the small bay of gdynia, opening out into danzig bay, on the baltic. it is 12 m. northwest of danzig, with which it is connected by the railway running westerly from that city to stolp and stettin. the polish govt., in building a railway to the port which will pass over territory entirely polish, has also a scheme for a canal to the port from a convenient point on the vistula, thus tapping the whole waterway system of poland. during the struggle with soviet russia in 1920, the poles were unable to utilise danzig for naval or military purposes, and this led them to build a port of their own. a suitable site was found at gdynia, where a fishing village of about 200 souls already existed. the depth of water near the shore varies from 10 to 20 metres; the bed of the bay gives firm anchorage; and the en- trance is sheltered by the peninsula of hela. the polish gevt. began the construction of the port in 1921, and by the summer of 1924 had built the southern mole, 550 metres in length, and a breakwater 175 metres long, forming part of the northern mole, together with a landing stage 150 metres long, a narrow gauge railway along the southern mole, an electric power station, water supply and other equipment. a contract for further construction was'signed on july 4 1924 by a franco- polish syndicate, the date for the completion of the contract being dec. 31 1930. its main provisions are (1) the making of an entrance canal, 1r metres deep, to the harbour, (2) the construction of the har- bour with a water area of about 4oo ac. and 1,060 metres of quays, together with a basin eight metres deep and a pier for passenger vessels, the depth of water at the quays being from eight to 10 metres and (3) the digging of an inner basin or dock on the foreshore, with a water arca of about 100 ac. and a depth of water at the quays of 10 metres. in 1924~5 the northern mole was ex- tended to a length of 700 metres, and considerable dredging and excavation work was done. in addition the polish govt. proposes to spend a jarge sum on the equipment of the port, com- prising large warehouses, a grain elevator, powerful cranes, rail- way sidings and paved roads. in 1930 the port will have quayage for 30 large steamers and a handling capacity annually of 2,500,000 tons, but it will be constructed so as to permit of a great increase in its size and capacity. in 1925, 85 vessels, of 73,351 net tonnage, entered, and 79 vessels, of 69,981 net tonnage, cleared the port, exclusive of coastal shipping. brstiograrhy.—s. slawski, poland’s access to the sea (london, 1925); the polish handbook, 1925, (london, 1925). (r. mac.) 160 geddes, sir auckland campbell (1870- _), british politician, was born june 21 18709, the son of auckland campbell geddes of edinburgh and the younger brother of sir eric geddes, and was educated at george watson’s college, edinburgh, and edinburgh university. he studied medicine, qualified as a practitioner, was at the london hospital for a time and later studied at freiburg. he was a demonstrator and professor of anatomy first at edinburgh, then at the royal college of sur- geons, dublin, and afterwards at mcgill university, montreal. he also had some military experience in the south african war and afterwards in the world war ror4-6. in 1916 geddes became director of recruiting, and in aug. 1917 minister of national service. a seat in parliament was found for him at basingstoke and he had little difficulty in accommodating himself to parliamentary life. after the armi- stice, sir auckland, who had been made a k.c.b. in 1917, became president of the local govt. board and minister of reconstruc- tion, and in may rorg president of the board of trade. latter capacity he began the removal of the barriers to british trade which the war had necessarily set up, and he had to deal with the difficulties which immediately arose in the coal industry. in this delicate task he was at least temporarily successful, and managed materially to reduce the price of domestic coal. in the same year an opportunity was afforded him to return to academic life by his election as principal of mcgill university. he accepted the appointment, subject to its not being operative till the abate- ment of the coal crisis allowed of his leaving the board of trade. but during the delay the government prevailed on him to accept instead the post of british ambassador in washington. his tenure of the embassy only lasted three and a half years (1920-3), but the time was crowded with important negotiations in which he showed himself a successful diplomatist. on quitting washington, sir auckland, who had been made g.c.m.g. in 1922, left the public service and became chairman of the rio tinto company. he married in 1906 isabella gamble, daughter of w. a. ross of new york. geddes, sir eric campbell (1875- ), british man of business and administrator, born in india sept. 26 1875, was the son of auckland campbell geddes of edinburgh, and the elder brother of sir auckland geddes. he was educated at the oxford military college and merchiston castle school, edinburgh. he gained some business experience at lumberingin the united states, and was afterwards connected with railways—first, the balti- more and ohio system, and then the rohilkhand and kumaton in india. returning to england he joined the north eastern railway co. under sir george gibb, and, having succeeded him in 1906, was himself the general manager of this line when the world war broke out. geddes was one of the business men whom mr. lloyd george, on becoming minister of munitions, enlisted in government employ. he became deputy director general of munitions supply, 1915-6, and was then appointed, though a civilian, director general of transportation, and succeeded in bringing the british lines of communication in france into a high state of efficiency. he was knighted in 1916, and in 1917 he was created k.c.b. and g.b.e., being transferred to the admiralty in may of that year as controller, in order to develop and utilise the whole of the shipbuilding resources of the country and concentrate them under one authority. a month or two later, in spite of having no parliamentary experience, he was appointed first lord of the admiralty, and was returned to the house of commons as m.p. for the borough of cambridge. after the armistice mr. lloyd george made use of sir eric’s powers as an organiser by commissioning him to co-ordinate government departments in regard to demobilisation. is success in these varied tasks was rewarded in jan. rgrg by the g.c.b. when the government was reconstructed in that month, he left the admiralty in order to organise and preside over a new ministry of transport. but in the autumn of 1920 there came the sudden break in trade, and it became evident that the country could not afford a department on the proposed scale. in 1921 a pill introduced by sir eric for the re-grouping of the railways was tn this | } | { geddes, sir a. c—genetics passed; he then resigned office in oct., and the ministry was re- duced in personnel and importance. sir eric himself was ap- pointed in aug. 1921 chairman of a small committee, later known as the ‘‘ geddes axe,” to recommend public economies to the government. in various reports in the winter of r921-2 the committee recommended savings amounting to £86,000,000; but sir eric complained that only £52,000,000 of this amount was actually saved. in 1922 he ieft parliament and returned to a business career, becoming chairman of the dunlop rubber co. and of imperial airways, ltd. he married in 1900 gwendolen, daughter of the rev. a. stokes, and had three sons. geddes, patrick (1854- ), british biologist and sociolo- gist, was born at perth oct. 20 1854. he was educated at perth academy, and in london and france, and became demonstrator in physiology at university college, london, in zoology at aber- deen and in botany at edinburgh. in 1883 he was appointed pro- fessor of botany at university college, dundee (st. andrew’s university) and in torg he became professor of sociology and civics in bombay university. as a biologist, in collaboration with james arthur thomson, he wrote the evolution of sex (1889; revised ed. 1901); evolution (1911); problems of sex (1912); and biology (1923), tending towards a new lamarckian position. interested in the synthetisation of science with art and history and its application to the conditions of modern life, especially in the direction of town-planning and housing, he travelled extensively, established a publishing house and pro- moted civic museums and industrial exhibitions. he was one of the leading members of the sociological society founded in 1904. his views are embodied in the papers of this society and in city development, a report to the carnegie dunfermline trust (1904), and cities in evolution (1915). he also wrote chapters in modern botany (1893) and life and work of sir jugadis c. bose (1920). in collaboration with victor branford he edited the series, (the making of the future, himself contribut- ing volumes, tie coming polity and, with g. slater, ideas at war. geiger, wilhelm (1856- ), german scholar, was born in nuremberg july 21 1856. he qualified as an academic lec- turer in 1886 in munich and became professor at erlangen in 1891. in 1920 he was appointed professor of indian and iranian philology at munich. he became the publisher of the zeit- schrift fiir indologre und iranistik, the zeitschrift fiir buddhismus and the grundriss dcr iranischen philologie. his chief works are: handbuch der avestasprache (1879); ostiranische kultur im altertum (1882); ceylon, tagebuchblatier und reiscerinnerungen (1898); litteratur und sprache der singhalesen (1900); dipavamsa und mahavamsa und die entwicklung der geschichilichen ueber- licferung in ceylon (edited 1905); pali, lilteratur und sprache (1916); pali dhamma (1921); elementarbuch des sanskrit (3rd ed., 1923); samyutla nikaya (eng. trans., 1924). geikie, sir archibald (1835-1924), scottish geologist (see 11.552), received the o.m. in 1914. ile died at his residence near haslemere, surrey, on nov. 10 1924. geikie, james (1839-1915), british geologist (sce 11.553), died in edinburgh march 1 1915. generator: see bdynamo. genetics (greek yeeveois, origin or creation).—this term was proposed at the third international conference on hy- bridisation, london, 1906, to denote the study of heredity and variation. in that sense it has been generally adopted, and by extcnsion is understood to include the physiology of reproduction and the art of breeding. though such inquiries have been pur- sued from the earliest times, the development of a special branch of science relating to them is recent. ‘the primary incentive was the hope that by applying accurate methods of observation and experiment to the course of heredity and variation a more precise knowledge of evolutionary processes might be acquired. modern theories of evolution are based on the assumption that species have arisen by descent with modification, and that the constancy and diversity which living things manifest in their reproduction provide a sufficient basis for that conception. it is significant that as a result of the preliminary work done under the new inspiration, attention has been largely diverted from genetics these more philosophical aims. beliefs current among naturalists, especially as to the nature and incidence of variability, were at once found to be widely incorrect. the scope and character of these discoveries are referred to below. their immediate conse- quence has been that the development of evolutionary theory is tacitly suspended or postponed, and activity is concentrated on the exploration of genetical physiology, the theoretical evalua- tion of the knowledge thus gained being relegated to the future. in these researches several methods of investigation are avail- able. modern genetics began with an attempt to observe em- pirically the course of contemporary variation from type; but though observations of this class have proved valuable in a pre- liminary survey, and have often been of use as indicating ma- terial for more prolonged investigation, the main advances have been accomplished by ecither (1) experimental breeding or (2) cytology. important sidelights on genetical problems have also been obtained through the study of developmental mechanics (entwicklungsmechanik) by experimental methods. experimental breeding the great stimulus to this method of research was given by the rediscovery in 1900 of mendel’s paper (sce 18.115). heredity, long regarded as a fortuitous and seemingly jawless phenomenon, was proved to follow regular principles which could in great measure be ascertained by experiments properly planned. a vast field was at once thrown open for investigation. mendel’s success was made possible by his genius for simplification. working with peas, he mace crosses between distinct varieties and watched the descent of their numerous characteristics, fixing his attention on each separately, and disregarding other differences. he then found that numerous distinctive features behaved in descent as if they were transmitted as units. these determining elements or units are referred to as factors or ‘‘genes’”’ (a term especially used by american writers, the equivalent of johannsen’s genen). the difierences determined by these factors can commonly be shown to be treated in hered- ity as pairs of alternatives or opposites, such as tall and short, coloured and colourless, hairy and smooth, each germ-cell being usually pure in respect of one or other of the contrasted char- acteristics. this is the principle of al/e/omorpitsm, and the membcrs of such pairs are called allelomerphs. ‘the zygote, formed by two germ-cells united in fertilisation, may be made up of two germ-cells alike in respect of any given pair, in which case it is said to be komozygous in that respect, or it may be a acterozygote if the uniting pair of cells are unlike. before the germ-cells of the heterozygote are formed a process of segrega- tion occurs, and there is a dissociation between the opposing elements introduced at fertilisation, such that the resulting germ- cells are again in normal cases pure in regard to each allelomorph. in respect of any such pair of differences the character of the heterozygote is sometimes intermediate, but it most often ap- proaches more nearly to one of the parental types. not infre- quently the approach is so close that the pure homozygote cannot be distinguished with certainty from the heterozygote. in mendcl’s terminology the character thus prevailing is a dominant, the other being recessive. the distinction is of importance anc ts the basis of certain suggestions as to the possible nature of al- lelomorphism. after the rediscovery of mendel’s work progress was rapid, and it was soon found that similar principles of descent apply to a great range of characteristics by which living things are dis- tinguished. the number of forms of life studied is now very large, and includes most of the kinds of plants and animals which are readily amenable to experiment or observation. man is evidently no exception, and we already know that certain features of humancolouration, especially of hair and eyes, and several congenital abnormalities are transmitted according to the mendelian scheme, some being dominant and others recessive. scarcely any satisfactory opportunities for studying the genetics of the lower plants (ferns, mosses, algae, etc.) have yet occurred, but one example has been described in a unicellular alga (pascher). of the features by which animals and plants are ioi distinguished most have now been shown to be dependent on segregable elements. reservation must be made in regard to differences which are simply quantitative, for there is a good deal of evidence suggesting that the elements by which size and weight are determined do not often form themselves into simple allelomorphic pairs. a similar doubt exists in regard to numerical or meristic distinctions. abnormalities —differences in instinct and other characters dependent on nervous mechanism are not, as such, distinct in their genetical behaviour, and some have been proved to depend on segregable factors or elements. in several breeds of fowls the hens are devoid of maternal instincts, and do not sit on eggs. this characteristic is recessive to the normal instinct, and segregation takes place in regard to it. the same is true of the pacing habit in horses as opposed to the trotting habit. the “ waltzing ” habit of certain japanese mice is recessive to the normal, segre- gates from it and breeds true when it reappears. this example is interesting, since the abnormality is almost certainly a con- sequence of deformity in the semicircular canals of the ear. descent in amlaun.—as to the descent of the normal mental attributes of man little is known with accuracy, but several ab- normalities of the nervous system are known to follow modes of descent which prove them to be subject to segregation. feeble- mindedness is a recessive condition which breeds true. paralysis agitans is also a recessive. hereditary chorea descends as a dominant; colour-blindness and a form of night-blindness may also be termed recessive (see sex). in heterozygous combination with the normal there is segregation, but the descent of these conditions is complicated by sex. it will readily be understood that though the determining factors may be transmitted as units, the distinguishing char- acters of animals and plants often must be due to the association of many independent units. of these, some produce their effects separately, but not rarely, though, independently transmitted, two or more unit-factors may be complementary to each other and combine to produce a joint effect or ‘‘ compound character,” as it is sometimes called. such complementary factors, if sepa- rately present in the organism without their complement, need not manifest their presence at all, and it is then only by breeding tests that their existence can be demonstrated. organisms may now be represented as aggregates of units which confer upon them their various attributes. the degree to which an organism may be thus resolved is as yet undetermined, but there is presumably a limit to the process, and it is natural to suppose that the detachable elements are implanted on a basis, which for a given type ts irreducible. reversion.—conceptions, formerly vague, now acquire an exact meaning. li’or example, reversion or ‘‘ throwing back” to an ancestral form, previously regarded as a mere caprice of nature, can at once be perceived to be due to one of two definite causes which operate with regularity. the reversion is either (a) the reappearance of a recessive characteristic, or (&) 1t 1s the conse- quence of the reunion of complenientary factors which, though © both present together in the ancestor, had been separated by variation and transmitted in distinct strains. for example, when a red-haired child is born to dark-haired parents the fact proves that the two parents are heterozygous in respect of the recessive red, which reappears when two germ-cells carrying it unite in fertilisation. moreover, if the statistics of a considerable number of such families of children were collected and added together it would be found that the proportion of red-haired was approxi- mately a quarter of the whole. the mere fact that one or both of the parents traces descent from a red-haired ancestor is not the cause of the reversion—for if either of the parents were homo- zygous in dark hair the red would not have reappeared. the reversion to an actual or supposed ancestral form conse- quent on the meeting of complementary factors is jess common in the ordinary practice of breeders, but is frequently seen in experimental crossing. when two white orchids crossed together give a coloured flower in f, or when a rose-combed fowl bred with a pea-combed bird gives chickens with the walnut comb of the malay fowl, the production of the unexpected colour or 162 structure is due to complementary action of two independent factors. but the old interpretation of the phenomenon as a con- sequence of such an ancestor having occurred in the pedigree is illogical and misleading. in the case of the walnut comb, for instance, it is quite posstble that either or both of the parent breeds never had a malay ancestor. the production of a new form by the meeting of complements should be regarded, like the properties of a chemical compound, simply as the empirical con- sequence of a certain combination of units, without reference to the previous history of those units. purity of ty, f greater importance, both theoretical and practical, is the fact that it is now possible to assign a precise meaning to this expression. to the pre-mendelian evolutionis purity was always a matter of degree, which might be gradually and, as it were, asymptotically approached in successive genera- tions of selection, but never actually attained. the practical breeder also has always regarded purity as a property necessarily dependent on a long course of selection. purity is now seen to be the condition of the animal or plant which is formed by the union of gametes bearing identical units. in respect of any allelo- morphic pair purity may thus be conferred, though in respect of other pairs of units the same organism may be impure, 2.c., heterozygous, or, in ordinary parlance, cross-bred. this is the central fact of mendelism, and on it genetics is based. the question of purity must therefore be considered separately for each pair of units. a thoroughbred horse, for example, may be pure in a number of characteristics which go to the making of the breed, but it may be impure in, say, colour. a chestnut horse, however, of whatever parentage, 1s pure-bred in colour, since that colour is the lowest of the series of horse colours, and chestnuts bred together give chestnuts only. by selection, the likelihood of producing purity is increased, but, as will subse- quently appear, no amount of selection can ensure purity. on the other hand, purity in respect of any character may be at- tained at once in any mating by which gametes of similar factorial composition happen to be brought together in fertilisation. from this proposition the corollary follows that the combination of two strains pure in any given respect will give a family uniform as regards the character considered, and the uniformity of such cross-bred families, especially when one of the parents contains few dominant factors, is in practice one of the simplest and most convincing tests of purity. genetic analysis.—by the institution of a serics of crosses with varietics and study of the composition of the succeeding genera- tions an aialysis of the factorial constitution of a given type can be made. ‘the numerical proportions or ratios in which the several combinations of characters are represented, the number of these terms in the series, and their respective genetical powers of transmission furnish the data from which the nature and number of the factors comprising the parental tvpe may he de- termined. in the carlier article on mendelism (sce 18.115) some of the simpler ratios and their significance are explained, but examples of a much higher order of complexity are often en- countered. the unravelling of these complications has led to some important discoveries. the many ways in which it may come to pass that two or more terms in a series of factorial com- binations may be indistinguishable from each other cannot be enumerated here, but a knowledge of some of the more signifi- cant causes of disturbance of what may be called the normal patios.(oiss sits or3243 2750101073 23:31,. ele.) is essential to a proper comprehension of genetics. cumulative faciors.—from certain crosses (especially of ce- reals) into which only one pair of differences had apparently been introduced it was observed (nusson-ehle; east) that the reces- sives reappearing in f*, were only 1:1:5 instead of the usual 1:3. investigation proved that from the dominant side feo factors with identical functions, though belonging to distinct pairs, had been introduced. consequently, among the dominants in f2 were some containing both these factors and others having one only. various results suggest that this multiplication, or better, accumulation, of similar factors is a common occurrence, and that the process may be extended in special cases. the ee body is promoted by sunlight. genitics inhibiting and lethal factors —many factors act by producing a negative result, inhibiting the development of some character, the determining elements of which are present though their action is not perceptible or largely diminished. of these the most easily demonstrable operate by inhibiting the formation of colour. the white pigment of the coats of animals and the feathers of birds, or of flowers, for example, is commonly due to the absence of the elements necessary for the formation of colour, but both in animals and in plints varieties have been found w hich are white, or nearly so, not through absence of pigment, but through thie presence of favors which, in some way not yet defined, inhibit the production of the coloured pigments. from some matings a mixture of white individuals may be obtained, which to the eye look alike or nearly so, though they represent various factorial terms and are genetically dissimilar. the process of inhibition may be carried much further, and there are well-established instances in which the animal or the plant cannot live if it is homozygous (containing two “ doses,” in popular terms) for a given factor. the classical instance of such /cthal factors, as morgan has called them, was met with in the breeding of yellow mice (cuenot; ip. m. durham). mice with yellow coats, bred together, give a majority of yellow, but always throw a proportion of some other colour—for example, chocolate or black. since in mice yellow is a dominant, it is clearly caused by a factor which the gametes cin carry. but the union of two gametes, both carrying this factor, does not give rise to a viable animal. it was suggested that two such gametes could not unite in fertilisation, but later work has practically proved that these fertilisations occur and that the resulting embryo perishes at an early slage (ibsen). the physivlogical action of the yellow factor in causing death is not known. in plants the “ golden ’ ’-leaved varieties are com- parable. they cannot breed true, but throw 2 yellow: 1 green. the purely yellow term is missing, and is clearly not viable (baur). the suggestion has been made that the yellow factor acts not merely negatively by diluting the amount of chlorophyll. but by inhibiting its formation, probably producing a body with this specifi¢ power. this 1s the more likely since golden varieties in dull weather turn almost a full green, whereas in sunlight they bleach to a full yellow, a fact indicating that the production of two doses of this factor kill the plant altogether, probably during embryonic life. linkage.—at an early stage in these inquiries it was observed that factorial units belonsine to separate allclomorphic pairs are not always distributed independently among the gametes of a heterozygote, but that some combinations occur regularly with a greater frequency than others. ‘the next step was the discovery that this zizkuge depends on the association of the linked factors in the parent from which the heterozygote was formed. for example, if a form ab is crossed with ae the normal expectation is that the double heterozygote aabb will form gametes -t b, a6, ab, ab in equal numbers; but if there is inkage between a and 8, then the parental combinations 4b and ab will be more frequently represented in the gamctic series than the other, or ‘ cross-over” combinations, 4b and a@b. but il the original cross were in the form ad x ab, then the most frequent gametes will be -{6 and a@b, the cross-overs, ab and ab being the rarer. this observation fon ns the starting-point from which modern genetical theory has been very kurgely developed. the terminology followed above is that introduced by t. ih. morgan, to whom progress has been especially due. it 1s some- times convenient to distinguish the case in which the two domi- nants {28 x ab) are introduced together by the parent as cou p- fing, and the converse (14x @b) as repuiston, but the physio- logical process is now recognised as being clearly the same in both cases, and there is no difference in the numerical proportions in which the parental combinations respectively reappear. it should be observed that the factors thus linked have phainly no connection with each other as regards the effects which they pro- duce in the zygote, but may concern the most dissimilar charac- ters. for instance, in the example first observed the linkage was that between the factor which makes the flower of the sweet pea genetics blue or purple (as distinguished from red) and that which makes the pollen grains long (as distinguished from round). according as the proportion of cross-overs is small or large the linkage is more or less complete. if both parental and cross-over terms are equally common there is no linkage. the most satisfactory test of the linkage-ratio is obviously provided by breeding the double heterozygote (:1¢—bdb) with the double recessive (aabd), and this mating should be carried out reciprocally since it is known that in plants (e.g., prinaaa sinensis) male and female sides of the same plant may show diiferent degrees of linkage (r. p. gregory), and that in animals (e¢.g., drosophila and the silkworm) crossing- over may be absent in onesex though occurring in the other. allelomorphism: multiple allclomorphs.— apart from linkage, segregation is always a separation of units affecting the same character, and from a very large range of observations it is possi- ble to represent the distinction between the allelomorphic pair as one in which a positive clement separates from a negative. in other words, allelomorphism may commonly be conceived as a difference which consists in the presence of something on the one side and its absence on the other. this conception is applicable whenever there is definitely pronounced dominance. it 1s natural that the characteristic which possesses dominance should be looked upon as due to the positive or present element, the recessive being the consequence of its absence. nevertheless, there is as yet no strict proof that this representation is physio- logically correct. for since we know that many factors may operate by inhibition it is always possible to invert the con- ventional representation and by putting negative for positive, to make a factorial scheme which equally agrees with the observed results. conventionally, for instance, the tall pea is represented as either 77 (homozygous) or jt (heterozygous), the dwarf being ff, from which the positive element 7 tallness is absent. but we cannot positively declare that the dwarfs may not be it homozygous in the presence of an inhibitor 7, whereas the tall plants might be either 71 heterozygous, or 71 homozygous in re- spect of the absence of this inhibitor. the significance of th's alternative mode of representation will be apparent when the application of factorial systems to evolutionary theory is at- tempted (sce mendelism). but when the heterozygote is inter- mediate between the two homozygous forms the ‘ presence-and- absence ’’ method of representation cannot be applied with any confidence. from the existence of such cases and from certain other considerations it has been urged, especially by american gencticists, that the method of representation by presence-and- absence is incorrect, and that a negative allelomorph should be treated as areal entity. there is no valid means of deciding this question as yet. the probability is perhaps that the absence should always be regarded as relative only. as a mode of sym- bolic expression the representation of the two allclomorphs as differing quantitatively is often convenient, though perhaps not universally applicable. types of allclomorphs—allelomorphism is, as the term imphes, a relation between two alternatives, and in any one zygote there can be no more than two. nevertheless, there are instances in which the same unit-factor enters into heterozygous combination with various alternatives in different zygotes, and each of these may thus be in allelomorphic relation with it. alternatives composing such a group of possibilities have been termed by morgan muitiple allelomorphs, and this expression is commonly adopted. its use, however, makes the application of the term “ multiple ” to “ factors ” in a totally different sense a probable source of confusion, and for this reason the word cumulative or some equivalent is there to be preferred as suggested above. the distinctions which together make up a set of multiple allelomorphs may commonly be recognised as a series of quanti- tative differences, the character aliccted being throughout the series the same. albinism in rabbits —one of the most familiar illustrations is provided by the degree of albinism in rabbits. the fully albino form is white with pink eyes, but there is a variety called hima- layan, which, though born white with pink eyes, acquires some pigment in certain parts. himalayan is dominant to albino but 163 recessive to the ordinary coloured types. if a coloured type is bred with himalayan the heterozygotes so raised cannot, when interbred, throw albinos, nor can heterozygotes raised from coloured x albinos throw himalayans, even though the albino used as their parent had itself been extracted from himalayans. the degree of albinism put in by the parents comes out in /y and in the same degree. hence it is not possible from similar parents to breed all three kinds, but on the other hand, each family can contain at most two of them. this phenomenon can be interpreted in either of two ways. the himalayan pattern may be regarded simply as a quantita- tive diminution or fraction of the sum total of colour needed to make the self-coloured type. the real albino is thus produced by the absence of the whole unit needed for colour, and the hima- layan by the absence of part of this total. tt is then obvious that the heterozygote, coloured x albino, could never produce a himalayan unless the colour-complex broke up again de nove. but on the analogy of the behaviour of other colour patterns the self and the himalayan might be conecived as cach consisting of two units: one for colour and one a factor determining its pattern, intensity or distribution. if there were a very close linkage be- tween each “ pattern,” factor and colour the observed facts could then be represented; but by continued breeding the sup- porters of this view would expect the missing cross-over to appear eventually as either a himalayan associated with recessive albinos or an albino associated with recessive himalayan. on the ground of simplicity the former view seems preferable. the significance of these two alternatives will presently appear. . study of grasshoppers —more complex illustrations of these possibilities have been described by nabours in certain grass- hoppers (puaratettix). the species studied presents a long series of colour forms, and experimental breeding showed that with certain exceptions all the pure forms behaved as if allelomorphic to each other. in other words, whichever two pure forms a and b were crossed together, the /, generation was -1 b, giving in f2 a family approximating to 14a:2.8:1bb. the whole series of colours is thus often described as a vast sct of multiple allelo- morphs. nevertheless, there are curious features in this case which raise a doubt whether this account is really correct. many of the distinctions are plainly gaantitative degrees in development of some one type of coloration whch are, as might be expected, allelomorphic to each other (cf. the himalayan rabbit): but among the clements comprising the total colouration of these grasshoppers there are several in which both the pigments and the positions they occupy are so distinet that the characters cannot easily be represented as determined by factors allelomorphic to each other. only by a very loose application of the term‘ colour’ can the distinctions be said to apply to the same character. hence, in this hitherto generally accepted illustration it seems probable that, in so far as the distinctions are actually quantita- tive differences in one respect, true allelomorphism may be recognised, but that the appearance of an allelomorphism be- tween factors of differing scope 1s more probably spurious and referable to close linkage (cf. haldane). no decision on this question can yet be made with ary confidence. llelomorphic contpicxes—among modern extensions of ge- netical theory none is more remarkable than the discovery that large and apparently miscellancous groups of characters are sometimes governed by clements capable of segregating collec- tively as a single complex. nevertheless, in the case of sex, we have long been familiar with one example. since the distinction between the two sexes in many animals is known to behave in segregation as if it depended on a single mendelian factor, we have to recognise that a number of distinctions of all kinds, structural and functional, may be treated in segregation as fac- torially single. in the special case of sex we krow further that particular genetic elements may be detached from the complex (e.g., the elements governing spur and broodiness in fowls, the beard in man, etc.), though the possible limits of such disinte- gration are unknown. ocnotheras.—renner’s experiments have shown that the in- heritance of the protean variations of several oenotheras is largely 164 effected by the transmission of similar complexes. each of these large composite factors or groups of factors (in so far as they prove to be divisible) may govern many characters of form, colour, habit, etc., and the whole group is transmitted as a single heritable entity. similar discoveries will probably be made in regard to other forms. the details are beyond the scope of this article, but it may be remarked that these complexes in oeno- thera supply one of the most striking illustrations of the phenome- non which may be called anisogeny (sce ‘‘ somatic segregation,” inf.) or the relegation of a factor or factors exclusively to one sex-side of a plant. for instance, whereas ocnothera lamarc- kiana, the species which provided de vries with his most cele- brated but unsound evidence of mutation, can be proved to be a permanently hetcrozygous form having two complexes equally distributed in segregation to both the male and the female gametes, the species biennis and many more, though similarly heterozygotes of two complexes, in segregation pass the whole of the one complex into the male gametes and the whole of the other into the female gametes. | the question whether the apparently simple factors which commonly behave as mendelian units are capable of further resolution is of much theoretical importance in its bearing on the problem of the nature of variation. such a complex factor as that which determines sex may evidently break up into simpler components, but for various reasons some geneticists incline to the belief that factors in general are permanent and irresoluble. whenever a series in f. derived from two clearly distinct and true- breeding types, consistsofanumber of intergrading forms, itis possi- ble to interpret this result as due to the operation of a multitude of originally distinct factors, or to the fractionation of some one or more of them. not very rarely in such series an extreme pa- rental type fails to reappear at all [e.g., the many-feathered tail of the fan-tail pigeon (morgan), or the long glumes of polish wheat from crosses with ordinary types. it is difficult to inter- pret the absence of the extremes simply as an indication of their statistical infrequency. the production of an innumerable series of colour-forms, as in the sweet pea, is almost certainly due to the fractionation of the colour-complex. until systematic crossing was undertaken, the extremes existed, but the intergrades did not. so also in drosophila, of which the normal eye js red, a profusion of intergrades ranging to the white eye, which was discovered first, has now appeared. though ‘ mutation ” is in- volved, the essential change is probably the disintegration or fractionation of the originally integral complex. cytological interpretations of genetic phenomena soon after the rediscovery of mendelian analysis the plausible suggestion was made that the behaviour of the chromosomes in the course of the maturation divisions was consistent with what might be expected if they were actually the bearers of segregable factors. since, however, the number of segregating factors in many forms far exceeds the number of chromosomes possessed. by those forms, it is clear that if the chromosomes are the carricrs of factors they must be capable of carrying many. the discovery of linkage, and especially. of the fact that linkage was determined by the parental associations of the factors, pointed in the same direction, for, as hinted (by punnett) in the earher article on mendelism (see 18.115), linkage or “‘ gametic coupling ” as it was then called might not unreasonably be supposed to be based on chromosomal association. thefirst development of this conception was made by t. h. morgan, whose investigations relating mainly to the fruit-fly drosophila, have inaugurated a new phase in the development of genctical theory. this insect is a subject unusally favourable for experiment inasmuch as it offers a profusion of variations or “ mutations,” and reproduces itself with great rapidity under laboratory conditions. the work began with the observation that the eyes, normally red, may be white, and that this variation is sex-linked, behaving genetically precisely as colour-blindness does in man. the white- eyed male mated with normal females produces offspring all normal. of these the sons cannot transmit the abnormality at all, whereas the daughters mated with normal males transmit the genetics white eye to half their sons. white-eyed females can only be produced as daughters of white-cyed fathers and all the sons of such females are white-eyed. supposing the male to possess an x-chromosome, this system of descent would be represented if it were assumed that in the normal the x-chromosome carried the dominant factor for red eye (see sex). the linkage with sex is thus found to be an expression of the association of the two determining factors for sex and red eye in the same chro- mosome. numerous other sex-linked characters were soon alter dis- covered, to which the same considerations apply, all collectively composing one linkage-group. the other factors identified in drosophila, amounting to more than a hundred, can all be represented as grouped in three separate linkage-systems which, with the sex-linked group, make four; and since from cytological observations the haploid number of chromosomes in this animal is also 4 the inference is drawn that the factors composing each linkage-group are borne in one chromosome. developing this conception, morgan suggests that the factors are arranged in the chromosomes as beads on a string, cach having a position normal- ly fixed in relation to the rest. crossing-over is thus represented as the consequence of an exchange of material between homol- ogous pairs of chromosomes in synapsis. (see cytolocy.) the pairs of chromosomes which then conjugate are with much probability regarded as respectively of maternal and paternal origin. the conjugating pairs seem to twist round each other, and occasionally there is (especially in amphibia) an appearance of anastomosis between them which is regarded as providing for an exchange of material between the homologous pairs, and thus for the formation of cross-overs. according as the linkage be- tween two factors is more or less complete it is supposed that the distance between the position of the two factors in the chro- mosome is smaller or greater, and in proportion as factors are placed close together the probability of their being separated in the process of twisting and anastomosis is regarded as diminished. the proportion of cross-overs is thus taken as a measure of the position of two factors in the chromosome. if a, b and c are three factors in the linkage group, and the closeness of the linkages between a and b and between b and c respectively be deter- mined experimentally, then from these two the linkage between a and c can be calculated, and the result of the calculation is commonly found to agree with the value found experimentally for that linkage. in this way the relative “loci ”’ of numerous factors have been determined with fair consistency, and the fact that this can be done forms a strong argument for the belief that in some way at least the factors must be disposed in linear sys- tems. that these systems are actually arranged along the lines of the chromosomes is as yet a matter of inference. attention must be called to the curious fact that in drosophila crossing-over never occurs in the males tn any of the four linkage- systems. as in most examples of sex-linkage studies, the linkage with the sex-factor is always complete; but all the other factors are liable to crossing-over in the female, though among the male gametes the original parental combinations reappear unchanged. conversely tanaka, examining linkages in the silkworm, ob- served that a pair of linked factors show crossing-over in the male, but not in the female, and the two facts together suggest some limitation of crossing-over to the sex which is homozygous in sex, the female in diptera, the male in lepidoptera. the develop- ment of the idea here outlined has become the subject of very active research and is described in a copious but somewhat esoteric literature, which can be followed only with difficulty by those not personally engaged in the work. that the outcome of these researches has led to a valuable codification of genetic principles is not in dispute; but until the main thesis, that the number of independent factors or of linkage-systems is never greater than the haploid number of chromosomes, has been shown to hold generally for animals and plants, this account of the nature of linkage, though probable, cannot be regarded as proved. the defect of the theory at the present time is that it rests on many subordinate hypotheses which are not all capable of independent verification. geneva the position of the factors, for example, is believed to be liable to changes due to the action of other factors, the effects of age and miscellaneous influences difficult to distinguish. errors of cell-division long regarded as the most probable source of varia- tion, may also cause disturbance. in two very remarkable in- stances it has been found possible to connect a disturbance in the normal course of heredity with a visible cytological irregularity— called by bridges “ non-disjunction.” in a certain family he ob- served that a sex-linked character failed to follow its normal dis- tribution to the sexes, and he was able to find that in this family the sex-chromosomes showed corresponding irregularities. in 192 he obtained similar evidence in regard to the fourth chromo- some and the group of genes attributed to it. thus a definite association between particular chromosomes and the transferable factors must certainly exist. giant forms.—the interrelation of genetical and cytolagical phenomena is further illustrated by the behaviour of ‘ giant forms.”” this name is applicd to certain varieties (chiefly of plants) in which the haploid and diploid numbers of chromo- somes are double those of normal forms. r. p. gregory bred such varieties of primula sinensis and found that in respect of various allelomorphs they might be quadripartite and not merely bipartite as the normals are. a plant, for example, might be drrr in colour or leaf-shape, and in consequence of the extra recessive elements, not distinguishable from the ordinary re- cessive, though in fact capable of throwing a small proportion of dominants. since recent cytological studies have shown that series of allied forms may contain various multiples of the lowest haploid number (chrysanthemum, for instance, having 9, 18, 27, 36 or 45) various extensions on these lines may be expected. somatic segregation.—in the genetics of plants a number of phenomena have been encountered which are difficult to reconcile with the view, otherwise not unacceptable, that the distribution of the factors occurs exclusively in the maturation processes of the germ-cells. apart from certain special conditions best known in variegated plants (which are sometimes irregular mosaics and sometimes consist of an outer ‘skin’ and an inner “ core,” dissimilar in their genetical potentialities) there are many plants in which the distribution of factors must have been laid down before the formation of germ-cells. e.r. saunders’s results proved that in certain stocks (afatthiola) the pollen all carried double- ness though the ovules were mixed in character, single and double. c. pellew showed that in the hermaphrodite campanula car- patica “ pelviformis ” the pollen bore exclusively femaleness and preponderantly white flower-colour (the plant being hetero- zygous for blue). the pollen of begonia davisii (a wild species with single flowers) carrics doubleness exclusively, and several similar examples are known, in all of which the segregation of characters must precede the maturation of the germ-cells. thus, while it is not a question that segregation depends on some cell- division, and very possibly on a differentiation of the chromo- somes, there is evidence that the cell-division in which this dif- ferentiation occurs must at least sometimes precede germ-forma- tion. as mentioned, in oenothera this “ anisogenous ”’ distribu- tion is exceptionally frequent. bearing on evolutionary theory.—this aspect of genetics can only be briefly treated here (see also mendelism). genetic analysis has shown that the appearance of variability as a con- temporary and widespread phenomenon is largely illusory. on studying a variable species critically it is found that the various forms cannot all produce each other as was formerly assumed, but that they stand in a regular descending order, being terms in a series of combinations of definite factors. such series are no evidence of contemporary variability. many of the terms can be separated in the homozygous condition, and thereafter may breed perfectly true. even such an appearance of variability as that seen in polymorphic species is frequently not above sus- picion of being the consequences of a cross, more or less remote. contemporary variation certainly may occur; but of the con- temporary origination of new species, or of the occurrence of genetic changes which can be colourably interpreted as likely to lead to the production of incipient species in a strict sense, no 165 indication has: been found. that the forms of life have been evolved from dissimilar precedent forms we know from the geological record, but as to the process by which this evolution has come to pass we are still in ignorance. all that can be said at present is that variation most commonly arises as an error of cell-division, and that it is quite conceivable that new species have so arisen. biblrograpny.—text-books: w. bateson, mendel’s principles of heredity (1913); ke. baur, finftihrung in die experimenitefle verer- buneslehre (114); yt. h. morgan, ieredity and sex (1915); the mechanism of mendelian [teredity (1915); the physical basis af iteredity (1919); r. c. punnett, mendelism (1919); ef. a. e. crew, animal genetics (4925). special references: l. cuenot, \" teredite chez les souris,” ee coal. exp. et gen. (1905); h. nilsson - ehle, ‘ kreuzungsunters. an hafer u. weizen,” lunds universitets ars- skrift (1909. and 1911); e. m. east, “a mendelian interpretation of a variation that is apparently continuous,” amer. nat. (1910); r. p. gregory, “ experiments with primula sinensis,” jour. gen. (1911); e. r. saunders, ‘‘ further experiments on the inheritance of ‘doubleness'’ and ‘other characters in stocks,” jour. gen, (igtr); r. p. gregory, ‘ ' genetics of tetraploid plants.” proc. roy. soe. (1914): r. kx. nabours, “‘ studies in inheritance and evolution in orthoptera, jour. gen. (1914 and 1917-8); c. b. bridges, ‘ non- disjunction as proof of the. chromosome theory of “heredity,” genetics (1916); a, pascher, “' uber d. kreuzung einzelliger, haploider organismen, \" ber, deut. bot. ges. (1916); ii. l. ibsen and fe, steig- leder, ‘ evidence for the ss in utcro of the homozygous yellow mouse,’’ amer. nat. (1917); pellew, ‘ types of segregation,” jour. gen. (1917): 0. renner, | ae itb. d. gametische konstitu- tion d. oenotheren,” zeits. f ind, abst. u. vererbungslehre (1917);3 w. bateson ant i. sutton, ‘ double flowers in begonia,’”’ jour. gen. (1919); engledow, ‘“ inheritance of glume-length and grain-leneth aw heat c ross,” jour, gen. (1920); j. b.s. haldane, ‘note on a case of linkage in paratcttix,” tour. gen. (1920); t. aida, “ inheritance of colour (sex-linked) in a plocheilus latipes,” genetics (1921). further references are y, ‘tanaka, “ occurrence of different systems of gametic reduplication in male and female hybrids,” zert. f. ind. abst. u. vererbungslehre (1915); t. h. morgan, “inheritance of number of feathers of the fantail pigeon,” «1 mer, naturalist (1918); c. b. bridges, ‘* triploid intersexes in drosophila melanogaster,’ ’ science (1921). the following periodicals are devoted to the subject: the journal of genetics (cambridge); genetics (menasha); genetica (the hague); jtfereditas (stockholm): zeitschrift ftir ae abstammungs- und vererbungslehre (berlin). (w. bn.) geneva, switzerland (see 11.587), had in 1920 a aeea ition of 135,050, including suburban districts. in the city proper tt was 56,292. geneva has been much changed in recent years, a large number of public and private buildings having been erected. the manufacture of motor-cars is a new industry, but it is as much an educational and intellectual centre as in the past. an historical and art museum was opened in the boulevard helvetique in t9g19, and various collections of arms, pottery, coins, pictures, etc., including those from the arsenal and the musee rath, have been brought together there. the musee rath, which was the international prisoners of war bureau during the world war, is now used for exhibitions. the imposing electoral palace was rebuilt with a large hall in 1913-6. a bridge to carry a loop line between the two railway stations has been thrown across the rhone below the junction with the arve. a reformation monument, begun in 1909 on the 4ooth anniversary of calvin’s birth, was unveiled in 1917; it represents a stretch of rampart, the wall of geneva with a narrow moat in front. in the centre are colossal figures of calvin and others, with statues on each side of eminent champions of the reformation, and bas-rcliefs and monumental inscriptions between. geneva was chosen in 1919 as the seat of the league of na- tions. the assembly met first in 1920 in the hall of the ref- ormation, built by public subscription in memory of calvin, near the grand quay. the league secretariat is housed in the palace of the nations, formerly a hotel, beyond the jetee des paques. in dec. 1925 this building was put up for sale, but as there were no buyers it was decided to retain it for the use of the league. a tablet in memory of president wilson was placed on the terrace wall in 1924. the international labour organ- isation was housed at first near the ariana museum, but for it a new building has been erected near parc mon repos. the office of the international red cross is in the rue de la prome- nade du pin, in the southeast part of geneva. 166 genoa (see t1.507), the chief port of italy and capital of the province of genoa, had a population of 316,217 in the municipal area of about 12 sq. m. in to2t. the industrial quarter of san pier d’arena on the west is now joined to genoa, and there is a growing residential quarter beyond the torente bisagno to the southeast, to which the via venti settembre gives dirce: access. large baile include the imposing banca a? italia in the via danie, and « building containing the bourse, the post office an:l a theatre in piazza de ferrari. the improvement of this piazza, the central square, was begun in 1913; blocks of houses have been demolished, and the area opened up. the historic palazzo san giorgio has undergone complete restoration, and is now the headquarters of the harbour board. a fine new sea front, the corso d'ttalia, adds considerably to the attractiveness of the new residential quarter. during the present century suburbs have arisen on the higher ground, and the via di circomvalla- zione a monte has been jaid out over the hills behind the city. the ee of the port is growing, and is considerably higher than that of any other in italy, representing almost one-fifth of the total tonnage of the country. exports are on an average hardly an eighth of the imports, which are chicily coal and cereals, and the marked drop which began in 1917 was due mainly to the decrease in coal. the doubling of the railway track to pisa and the development of the stations at san pier d’arena and at genoa brignole, about two m. east of the principal station, with the extension of electric power to the main line towards turin and along the coastal fae: have improved the port’s facti- ties. a scheme for the extension of the harbour, taken in hand in 1922, included the new vittorio emanuele and san pier d’arena basins. shipbuilding is increasing in importance, and there is a considerable motor car industry: the manut: icture of hats is also a growing trade. . genoa, conference of, (april to-may 19 1922,) a meeting df representatives of the british sclf-governing dominions and of 29 european states, including not only the allies and ex-neutrals, but all the ex-enemy powers except turkey (who was excluded on the ground that she was an asiatic country). above all, the conference was attended by representatives of soviet russia, and the dominant issue was the renewal of relations between russia and the countries of europe. the principal result achieved, however, was one which the organisers of the con- ference had not intended, namely, the signature at rapallo, on april 16 1922, of a separate treaty between soviet kussia and germany. the prospects of the genoa conference had been somewha clouded, before it assembied, by the wave of anti-liberal feeling in france, which declared itself politically in the superseding of m. briand by m. poincare as prime minister. since the project of the genoa conference had already been accepted by the supreme council, including the representative of france, before m. poincare came into power, he could not reject it altogether, but he took every possible step to interpret the agreed programme in the narrowest sense, and to hedge the participation of russia with the fullest possible restrictions, he refrained from attending the conference but enforced his policy by giving detailed and stringent instructions to his representative, mi. barthou. the general conference was preceded by a meeting between m. poincare and mr. lloyd george at boulogne on feb. 25, a meeting of allied economic experts in london from march 20 to 28 which drew up detailed agenda for genoa, and two other preliminary meetings of a regional character, one between the members of the little entente at belgrade, and another at warsaw between poland, latvia, estonia and finland. the parties represented at warsaw subsequently conferred at riga with representatives of soviet russia. the invitation to genoa was bee pete by the soviet govt. with alacrity, but was declined by the united states. adverse influence of the rapatlo treaty.—at its first plenary session the conference set up four comrnissions, the first to exam- ine methods of putting into practice the principles of the heen cs resolution of jan. 6 1922 (see cannes, confrrence o f), while the other three were to deal respectively with financial subjects, genoa—gen tile economic and commercial subjects and transport. these three latter commissions, which were concerned with technical ques- tions, all reported before the conference came to an end; but their reports were bound to remain academic unless the first com- mission achieved positive results. the task of the first commis- sion was more difhicult, because it was general and political in character, and its work was soon suspended in favour of informal discussions between the three principal allied powers and belgium on the one side, and the russians on the other. vhe germans, who felt themselves left out in the cold, retorted by negotiating their separate treaty with the russians at rapallo, in which, at least on paper, the objects of the genoa conference were achieved as between these two parties by a mutual re- nunciation of reparation claims and a resumption of normal con- sular and diplomatic relations. hawever, this separate russo-german treaty damaged the general prospects of the conference by the shock which it gave to the allies, and especially to belgium and france, who, ever since the armistice, had been oppressed by the nightmare of a military alliance between germany and russia. the rapallo treaty seemed like a first step in this direction, even though the published text contained no military clauses, and the allies addressed a series of acrimonious though ineffective notes on the subject to the german delezation. in these circumstances there was little prospect of success for a general pact of non-aggression, which mr. lloyd george suggested on april 25. meanwhile at the very first session, m. barthou had had a hostile encounter with the principal russian delegate, m. chicherin; but the con- ference actually broke down through the intransigence of bel- gum. when an attempt was made to secure a4 common draft of proposals to be presented by the european delegates to the russians, the bel:sians insisted upon the integral restitution of foreign-owned private property in russia. m. barthou sup- ported the belgian contention, and was himself supported in this attitude by m. poincare. eventually a formula on the british lines was carried over m1. barthou’s head, even belgium finally giving way, but it was so evident that, with great britain and france divided, no positive result could be achieved, that the genoa conference was quietly wound up by remitting its agenda to a mixed commission of experts. this conference of experts duly met at the hague from june 26 to july 20 1922, but it foundered, like the genoa conference, on the rock of forcign- owned private property in russia, in regard to which the russian and the franco-belgian views again proved incompatible. the reconstruction stand point—al|lthough the genoa confer- ence led to no positive results, it was interesting as the first gen- eral european conference after the war of 1914-8, and because economic and financial problems were approached from the point of view of reconstruction, and not of reparation. it was also interesting as the first attempt at a settlement between the european governments and sovict russia. at genoa the difh- culties which proved crucial on later occasions were already encountered. ca tie) gentile, giovanni (1875- ), italian philosopher and politician, was born at castelvetrano (trapani) may 29 1875. hic studied literature and philosophy at the university of pa- lermo, and after a series of university appointments became in 1918 professor of the history of philosophy in the university of rome where he founded and became director of the school of philosophy. ‘t'wo years later he founded the giornale critico della jilosofia taliana. created a senator in 1918, he supported the fascist movement from its beginnings, and when mussolini’s govt. came into power, gentile was appointed minister of edu- cation. iie was then a liberal, but subsequently joined the fascist party, of which he became a prominent member. as min- istcr of education ke carried cut an organic reform of the italian educational system, abolished the traditional shibboleths of positivist and materialist teaching, and suppressed many invet- erate abuses. he introduced the system or state examinations and also the teaching of religion into the elementary schools. the universities wholly maintained by the state were limited to 19, while others, maintained privately or by local administra- geodesy tions, received state subsidies if they complied with certain standards. among gentile’s numerous works are: // mrodernisnio ei rapporti tra religione e filosofia (1909), i problemi della scal- astica e il pensicro italiano (1913), j fondamenti della filosofia del diritlo (1917), le origini della filosofia contemporanea in italia (4 vol., 1917-23), id problema scolastico det dopo guerra (1920), la riforma dell? cducazione (1920), lt pascismo al governo della scuola (1924), che cosa e il fascismo (1925), ete.",
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