ATLANTIC CITY, N.J., U.S.A

growing slowly in permanent population and the normal occupations of an american city, atlantic city continues to develop its unique position as an all-the-year-round playground for the nation and a favourite meeting-place for conventions. the permanent population in 1920 was 50,707, of whom 10,946 were negroes and 7,009 foreign born; in 1925, according to the estimate of the census bureau, it was 53,287. this is increased to 300,000 in aug. and it is estimated that there are 15,000,000 visitors in the course of a year. in 1925 there were about 1,000 hotels. the atlantic city airport, dedicated in may 1919, was the first in the world. during the summer there is an aeroplane passenger service to new york. the boardwalk, built of steel and concrete with a board flooring, 60 ft. wide in the central portion and ex- tending 8 m. along the ocean front, is connected with six great recreation piers reaching out over the ocean for 1,000-2,500 ft. and lined with sumptuous hotels, shops and places of diversified amusement. the city issued bonds to the amount of $1,734,000 in 1924 for the purchase of a convention hall site bordering on the boardwalk. in 1912 the commission form of government was adopted. ts 4 atmospheric electricity (sce 2.860).—our knowledge of atmospheric electricity at the earth’s surface depends on data from comparatively few stations. of the air we have only scanty information from balloon observations. thus any mean results put forward are exposed to much uncertainty. in the article tn vol. ii., page 864, 125 volts per metre was accepted as a mean value p of the potential gradient at. ground level. this implies the presence on the ground of a negative charge whose surface density 2 in electrostatic units (e.s.v.) is given by : — 4mg = p= 1-25/300=0-00416 taking the earth as a sphere of radius 64 x 10’ cm., its entire negative charge is -o-o00416 x 4 x 107” e.s.v. or —56 x 104 cou- lombs. the inference drawn (2.867) from the rapid fall in the potential gradient with increased height above the ground shown in balloon ascents was that an equal positive charge exists in the atmosphere, most of it in the lowest 4 kilometres. the charge at any spot on the earth is undergoing constant change. a vertical air-earth current is continually passing. this has been observed 258 at only a few stations, and at most of them only during fine weather, and at one or two hours of the day. from these observations it has been inferred that the fine- weather current is directed downward, and averages about 2 10°! amp. per cm’. under a uniform current of this magni- tude, the earth would receive in one second about 1,030 coulombs of positive electricity, and its entire negative charge, if unre- plenished, would disappear in about nine minutes. a limited area on the earth may experience uninterrupted fine weather for weeks, and if it were an electricity tight unit, we should assume that it also received a liberal allowance of some very pene- trating radiation which ordinary instruments do not record. we know, however, that the earth is a conductor. thus the phenomena might be accounted for if regions experiencing bad weather (during which potential gradient is often negative) received an excess of negative electricity. during bad weather, electricity passes between the air and the earth in at least three ways. there is an air-earth current, carried as in fine weather by ions; there is electricity brought down by rain or snow and there is lightning. our information as to these supplies will be discussed presently. besides the large rapid changes of local earth charge, asso- ciated with rain and lightning and the passing overhead of charged clouds, there are slower regular changes depending on the time of day and the season of the year. in britain, potential gradient is higher in the evening than in the early morning, and higher in winter than in summer. if this held good everywhere, the charge on the earth as a whole might remain invariable (fine weather and bad weather effects balancing out), electricity pass- ing between areas in which it is early morning and areas in which it is evening, and between areas where it is summer and areas where it is winter. but doubts are entertained as to whether the phenomena in the southern and northern hemispheres are com- plementary, a maximum potential gradient having been ob- served in summer in some southern stations. the view has also been put forward that the diurnal variation at sea, of which our observational knowledge is very scanty, is mainly dependent on universal, not local time. again results from some stations have been supposed to show an increase in potential gradient with increasing sun-spot frequency. if these conclusions are correct, the charge on the earth as a whole would seem to have a diurnal, a seasonal and an 11-year period. a knowledge of the mean value of the potential gradient, therefore, is of fundamental importance, and here a serious difficulty at once presents itself. the earth’s surface is not in general, an unbroken level plain. in the hollows or in the immediate neighbourhood of trees, the potential gradient is inevitably lower than elsewhere. the‘surface charge is not uni- formly distributed. there is more of it on ridges than in hollows, and it exists on tree tops rather than on the ground underneath. but over an area of many square miles, under normal conditions, the charge should be at least approximately the same as if the surface were flat. what we really require to know is the potential gradient over a flat area making as close an approach as possible to an infinite plane. also the potential must be measured in such a way that the presence of apparatus and observer is without effect. a suitable spot is hard to find near many stations, and the factor for reducing observed values to the infinite plane is seldom a quantity wholly above suspicion. in the case of many of the older data, no factor was applied, and the results are only of relative value. a factor is known to have been applied in the case of the following selection of more recent values of potential gradient p:— ete table i.—mean potential gradient for the year p (volts per station period latitude metre) eskdalemuir 55°n iqi2-22 256 potsdam 52° n ‘1904-23 203 kew 51° n 1913-23 333 davos 47° n 1909-10 64 tortosa 41° n 1911-23 108 samoa... 14°s 1913-8 . 115 cape evans 735 iqii-2 87 the above value for potsdam is lower, and that for kew higher than the older values given in. 2.861 and 862. this is atmospheric electricity mainly, 1f not wholly, due to a revision of the reduction factors. the exceptionally low value at davos, it is suspected, may arise partly from the position of the site of the absolute observations in a narrow valley. but, as a rule, we should expect imperfec- tions in the site or in the method adopted to lead to too low a factor, and so to an underestimate of potential gradient. on the other hand, potential gradient data are mostly from fine weather days. an attempt to estimate the effect of this restriction at kew was made for 1922. on the average day, means from the whole 24 hours and from the four hours 3h., oh., rsh. and a2th., are closely alike. the electrograms were measured at these four hours on all days of complete trace, and the means so obtained were as follows: all days 271 volts per metre, occasions free from negative potential 293 volts per metre. the corresponding all day mean from selected fine days was 318 volts per metre. this suggests a reduction of some 15% if mean annual values at kew were derived from all days instead of from selected fine days. at kew during 1923, a fairly normal year, negative potential was recorded on 190 clays, but its longest duration on any one day was 12-5 hours, and its total duration for the year only about 350 hours. potential gradient seldom remains con- tinuously negative for long. during rainfall, alternations of sign often occur, and during thunderstorms they are too large and rapid for the ordinary apparatus to follow accurately. thus disturbed weather results are affected by considerable uncer- tainties. there are other sources of uncertainty. few observatories are in sparsely peopled areas. many are in or near large towns, where the atmospheric conditions suffer from by-products of civilisation, which unfortunately influence atmospheric elec- tricity. the breaking up of water, whether by natural or arti- ficial means, leads to a separation of electricity. condensed steam usually carries a positive charge, and steam trains may exert a considerable influence on potential gradient. but there is another more widely spread influence, atmospheric pollution mainly due to smoke. in a smoky atmosphere (for example in dublin, where special observations have been taken) light, mobile ions diminish, and heavy, slow-moving ions largely increase in number. the conductivity of the air tends to fall, and the potential gradient to rise. the practice at kew observ- atory is to derive mean values and diurnal inequalities of potential gradient from 1o selected fine weather days a month. the 10 selected days of each month of the two years 1921 and 1922 were divided into a more dirty 5 and a less dirty 5. the mean amount of pollution in the atmosphere, as measured by the owens pollution recorder, and the mean potential gradient, were calculated from the two groups of days separately, for the year as a whole, summer (may to aug.) and winter (nov. to feb). table it. gives the results, the pollution being in milligrammes per cubic metre and the potential gradient in volts per metre. table ii.—potenttal gradient on days of greater and less atmospheric pollution year summer winter pollut. 0-491 0-254 class of days potent.| pollut. | potent.| pollut. | potent. 343 252 155 more dirty . less dirty. ideal (clean) 0-252 0-139 the results for the ideal clean day were calculated on the hypothesis that the increase of potential gradient due to pollu- tion varies directly as the amount measured by the owens recorder. the atmospheric conditions which promote air pollu- tion at kew, would possibly lead to enhanced values of potential gradient even if the atmosphere were pure. but it is hardly open to doubt that pollution has a direct effect. in 1921 when a pro- longed coal strike led to an exceptionally clear atmosphere in england for several months, the mean value of the potential gradient at kew was the lowest recorded for many years, being only 281 volts per metre, as against 315 volts per metre in 1920. and 318 volts per metre in 1922. atmospheric electricity sunspot influence, such as is prominent in the diurnal range of the magnetic elements, has been supposed by some to apply to the mean annual value as well as to the diurnal range of the potential gradient. a priori the result is probable enough, in view of the in- creased conductivity of the upper atmosphere which auroral and magnetic phenomena suggest at sunspot maximum. the normal way of investigating the existence of a sunspot influence on the mean yearly value q of any quantity, is to apply wolf's formula q=a-+bs, where s is the sunspot frequency, or area, and a, and b constants, to be determined from the observed values of q by least squares. a is the value of qo when s vanishes, as it nearly docs at sunspot minimum; b is positive or negative according as q is greatest or least at sunspot maximum. in the average 11-year period the extreme value of s approaches 100; thus 100 b/a is a convenient measure of the importance of the sunspot influence. any haphazard collection of figures will naturally give a finite value for b. to determine wheth- er the sunspot influence is real or imaginary, we consider the size of the correlation coefficient r (regarded here as an essentially positive quantity, the sign of the correlation being shown by 8). for perfect correlation 7 is unity, and unless r is a substantial fraction the cor- relation is unlikely to be real. table iii. gives some data obtained for recent periods of years, the only case in table ii]. where r is as large as is usual in the case of the magnetic daily range, is that of the mean yearly value at eskdalemuir. the values of 100 b/a are all small compared with those encountered in terrestrial magnetism, (g.v.). on the whole, the results for 1911-21 at the three stations favour a small 11-year variation in the mean annual value of potential gradient, with a maximum at sunspot maximum and a fourth station, tortosa, not included in table iii. also favours this view. the small values generally obtained for r throw, however, doubt on the reality of the sunspot influence. if it is real and universal, as in the case of mag- netism, it is clear from the potsdam results that it may be masked at individual stations by other causes. the verdict perhaps most in accordance with the facts is ‘‘not proven.” 2.861 contained a good many data for the annual variation of the potential gradient. table iv. gives some more recent data in absolute measure. under w/s is given the ratio bourne by the mean value of the four winter months (nov. to feb. in the northern hemisphere, may to aug. in the southern hemisphere) to the mean value of the four summer months. | all the five stations in the northern hemisphere show the max- imum in winter, the minimum in summer. this is equally true of older data from these and a number of other northern stations. but the opposite phenomenon, according to b. chauveau, has presented itself at one exceptionally high level station in france (pic du midi, 2,860 metres) ;though this is strongly opposed by the results at davos, the phenomena on mountain summits want further investigation. for the southern hemisphere, results are conflicting. only one year’s data are available for buenos aires, but unless it was an altogether exceptional year, winter is clearly the period of maximum. at samoa, earlier data from 1906-8 placed the maximum clearly in winter, but these are now under suspicion. the samoa data in table iv. though derived from 6 years, depend really on only 40 months, there being many breaks in the record. the annual variation they 259 suggest is very trifling and uncertain, a result not improbable in itself in view of the low latitude. the observer at cape evans, dr. g. c. simpson, pointed out that the maximum appears in summer in all the observations taken in high southern latitudes, including hut point (78° south), belgica station (71° south), port charcot (65° south) and petermann i. (65° south), and expressed the opinion that “the potential gradient in the ant- arctic is higher in the summer than in the winter.” some authorities have gone further, claiming that a maximum in sum- mer is characteristic of the whole southern hemisphere. this seems, however, to be supported by results from only two sta- tions outside the antarctic, viz.: cape horn and batavia. the observations at cape horn were of very limited duration, and at batavia results for different periods, from different levels, appear about as conflicting as at samoa. older observations at melbourne agree with those at buenos aires in placing the maximum in winter. the conclusion that the seasons influence the potential gradi- ent in different directions in the two hemispheres, the maximum appearing in both near northern mid-winter, cannot be accepted as proved on existing evidence. it is a question obviously of fundamental importance. : the diurnal variation of potential gradient at a number of stations is considered in detail in recent works by chauveau and mathias. the former, who has made a special study of the subject, thinks that the normal type of the diurnal variation makes a close approach to a single 24-hour wave, the maximum appearing in the evening, the minimum in the early morning. the double oscillation generally encountered, with minima in the early morning and early afternoon, he regards as largely the product of local disturbing causes, such as atmospheric pollution, trees, heating of the ground by the sun. the expediency of reserving judgment will appear on considering figs. 1 and 2. results for kew (1913-23) and eskdalemuir (1912-22) appear side by side in fig. 1. winter includes the four months nov. to feb., summer the four months may to august. the heavy horizontal lines answer to the mean values for winter, the whole year and summer. the eskdalemuir winter curve suggests chauveau’s ideal station. its afternoon minimum and forenoon maximum are inconspicuous as compared with those at kew. but the eskdalemuir summer curve ts of quite a different type. the morning minimum is hardly represented, and the afternoon minimum is more prominent even than the kew one. eskdale- muir is in an upland valley with pure air and few trees. fig. 2 illustrates the effects of atmospheric pollution in an actual case. the curves show the diurnal variations for the year as a whole of potential gradient and atmospheric pollution at kew. they are based on the two groups of more dirty and less dirty days of the two years 1921 and 1922, already described. the unbroken lines refer to the more dirty, the broken lines to the less dirty days. the line oo represents zero potential and zero pollution. the table iil.—values of constants in wolf's formula nature of q station period a b 100 b/a r v/m v/m potsdam 1904-23 211 —0:216 —0-102 “34 : kew —— 1904-23 309 +-0°303 -++0-098 “37 mean value of potential gradient eskdalemuir . i9li-2i 236 +0-440 +0-186 83 potsdam iqii-21 205 —0o-143 —0-069 46 ; ; kew od iqii—21 312 +0:444 +0-142 56 range diurnal inequality for year eskdalemuir . 1912-21 110 +0-115 +0-105 25 kew i91i~21 152 —o-i1i4 —0-°075 23 table iv.—annual variation of the potential gradient (volts per metre) period 1912-22 1904-23 1913-23 1908-i0 1918-23 1913-8 i9ii-2 iqii-2 station eskdalemuir buenos aires cape evans 260 scales were so chosen that the horizontal lines representing the mean values for the more dirty group of days (viz., 343 vm and o.490 milligrams/metre *) come at the same level. the mean values for the less dirty group (252 v/m and 0.252 milligrams/metre 3) are also shewn by horizontal lines. with in- creased pollution there goes increased potential, and the diurnal variations of the two elements are of very similar type. but while the afternoon maximum and minimum of pollution become mid- mid- mid- / night night night mid- 18 night hours 6 hours noon 6 18 hours noon hours 300 vin " ie ia ay | b\' gh a 200 yim a bh aa ba fa b | | at a 100 eskdalemuir yeareaa= summed? esccses winter ann fic. 1.—graph comparing the diurnal variation of potential gradient at kew and eskdalemuir. relatively less important as pollution diminishes, this is not true of potential gradient. the electricity brought down by rain or snow has been measured by the following amongst others: g. c. simpson (simla 1908-9), k. kihler (potsdam 1g08), schindelhauer (potsdam 1909-11), a. baldit (puy-en-velay 1910-11), mcclel- land and nolan (dublin 1911-2). simpson obtained a complete record during the rainy seasons of two years. kahler and mid- 6 18 = mid- mid- 6 ; 18 mid - night hours noon hours wight night hours noon fours might aa et hh teh oe a milligrams/m. 400 [ ym. re pe i a 8 b | b a 8 a a w a a bs { 1! i a a b | a a ¢ “sc 5 ee milligrams,‘ dene aves 300 i - “an. ium 3 0-4co a! seen. ' milligrams/m. rn naueh a te ho r covenn 0300 , 200 f3 neeaen milligrams/m. “mf reece sgsn08 pte) feet a bersegss es 0200 , g breegrer aaa: milligrams /m. r bershase ig r aecanuee 100 peel eee oh yin tl askesoes we ihe h pee 0-100 aa on milligrams fn> i as eid an bil | | | a a a8 € a as @ ib ol ar b 0 potential gradient atmospheric pollution more dirty days mmm less dirty days —-_ fic. 2.—graph illustrating the diurnal variations of potential gradient and atmospheric pollution for a year, at kew. atmospheric electricity schindelhauer had also continuous records. these three ob- servers measured the results of two-minute falls. baldit took 15-second falls, but his observations being taken by eye, are in one way less complete, while giving more minute details of individual cases. on some occasions, as observed at dublin, the charge brought down by rain, especially by fine rain, may be of one sign for a considerable time. but in general, there are frequent changes of sign and continual changes of intensity. thus even short intervals of time tend to give too low maximum rates. in table v., a denotes the ratio of the duration of positively charged rain to the duration of negative, b the ratio of the total quantity of rain charged + to the total quantity charged —, c the ratio of the total positive charge brought down by rain to the total negative, d is the excess of positive over negative charge cal- culated for a total rainfall of 100 centimetres. table v.—ratnfall electricity data place period | kindof rain| a b simla 1908-9 j all 2°5 3:2 | 1765 potsdam 1908 - ad 2 5-6 . 1909-ti so ks tey 2°83 dublin . iqii- 12 x a1 | 4:8 puy-en-velay | 1911 a 29 ~ i-4 a . | i9ii ordinary 5°31 43 | 23 “ iqii storms 1-7 | 1-5 | 1-2 ‘ igil squalls i-r | 1-2 | 1-1 at dublin, little if any of the rain that fell was accompanied by thunder. the observers at potsdam and puy-en-velay drew a distinction between ordinary rain, with which they found a marked excess of positive charge, and thunderstorm or squall rain for which the excess of positive was trifling. much of the rain at simla was storm rain, and it was generally heavier than elsewhere, but it gave as marked an excess of positive as did ordinary rain. all the observers found the charge per cm of rain to be greatest in the lightest rains. in one 15-second fall at puy-en-velay, the charge per cm? was —43.6 e.s.u. this formed part of a 5-minute fall for which as a whole the charge per cm? was —37.7 e.s.u. negative charges exceeding 40 e.s.u. per cm’ were observed at potsdam on one or two occa- sions. the greatest positive charge per cm® observed was +-26.7 e.s.u. at puy-en-velay. while the charge per cm?’ tended to be greatest in light rains, the quantities of rain then occurring were so trifling that the largest vertical currents due to rain occurred during the heavy storm rain. at puy-en-velay, currents were measured of — 1-10 x 107 and + 1-05 1072 amp. /em2, and currents nearly, if not quite, as large were also observed! at simla. the figures supplied by simpson give as the average current in amps. per cm? for the positively charged and the negatively charged rain, the respective values +41 x107% and —31xx107%, the corresponding figures for puy-en-velay are +20%1075 and —42x107'5. no charge was recorded during some 40 per cent of the time of sensible rainfall at simia. allow- ing for this, the average current during sensible rainfall was at least 2107! amp./cm?, or 100 times the ordinary iton-carried current in fine weather. the excess of positive charge observed at simla was +15-1 e.s.u. per annum. this is sufficient to maintain the fair weather current, estimated at 21078 amp./cm?, for 290 days. the excesses of positive electricity observed at potsdam and puy-en-velay are much tess, and considerable differences were ob- served at these stations at different seasons of the year. also, while snow on the few occasions when it was observed at simla brought with it a considerably larger positive charge than the cor- responding amount of rain, the charges accompanying snow at pots- dam appeared to be small and predominatingly negative. no simple explanation of thunder and lightning can be accepted without reserve. but there appears a general consensus of opinion that the theory proposed by dr. g. c. simpson is less open to criticism than any other. it depends essentially on the following facts: 1. the velocity of raindrops falling through still air, increases with the size of the drop only up to a certain diameter. drops of larger size tend to flatten and break up into smaller drops. 2. the breaking up is accompanied by the sep- aration of electricity. 3. during thunderstorms there are in limited areas rapidly ascending air currents. it is inferred that when a larger drop falling through an ascending air current breaks up, the drops into which it breaks receive a positive atmospheric electricity charge, the negative going to the air, and are carried up. they may unite and increase in size, fall and get broken up again. in this way a continual separation of charge may go on, until the potential is locally raised to the point at which a lightning stroke becomes possible. for information as to the changes of potential gradient during thunderstorms; we are indebted mainly to the work of c. t. r. wilson at cambridge and h. norinder at uppsala. wilson’s method consists essentially in measuring the alterations in the charge on a plate representing part of the earth’s surface. norinder measures the potential difference between two insulat- ed horizontal wires at different heights. accompanying a flash of lightning there is a very rapid, if not quite instantaneous, change of potential gradient. out of 864 flashes observed by wilson 528, or 61%, were accompanied by a rise of potential gradient. wilson supposes a thundercloud to consist essentially of a lower and an upper part, oppositely charged, the sign of the upper charge being usually positive. supposing then a charge +q, to be concentrated at a height h2, and a charge —q, at smaller height h, on the same vertical, we can calculate the potential from these charges and the image charges +q» and —q, at depths hz and h. from this potential, we easily obtain the following expression for the dow nwardly directed vertical force at ground level 2q0.h.(h2+l?) 3? — 2q,hi(h2+l?) 3 where l is the horizontal distance of the spot where the force is measured from the vertical line on which the charges are sup- posed collected. immediately under the charges l vanishes and the force is given by ~2 (q, hi — q.h») as hp considerably exceeds h; this is negative—implying a posi- tive charge on the ground—unless q: is much larger than q:. at a distance l, large compared with ho», the expression for the force becomes approximately 2 (q2he a 0,5,)l= this is positive unless q; is considerably larger than q.. thus at a large distance, the sign of the vertical force and the sign of the earth’s charge are the opposite of what they are immediately under the cloud. supposing q: positive, when lightning passes discharging the cloud, potential gradient will suddenly rise under the cloud, while at a large horizontal distance it will fall, the fall varying inversely as the cube of the distance. according to wilson’s observations, the change of potential gradient docs follow roughly the law of the inverse cube. wilson’s theory does not give q directly, but qh, where h is a sort of centre of gravity of the thunder cloud. assuming h to be 2 km wilson found q to be on the average, about 20 coulombs. supposing the value correct, the initial value of the current recharging the cloud must usually amount to several amperes. for a spark to pass in or- dinary air at atmospheric pressure, a voltage of 30,000 v/cm is requisite. this would actually be found at the surface of an isolated sphere of 250 m. radius charged with 20 coulombs. the presence of the earth and the supposed bi-polarity of the cloud, of course complicate matters. wilson observed changes of potential gradient of the order 1,000 v/m for flashes at ro km distance, and inferred they might be of the order roo,ooo v/m under the centre of the cloud. norinider has actually observed changes of potential gradient of 200,000 v/m. under excep- tionally high potential gradients, air tends to become ionised, and the presence of pointed objects like grass and leaves pro- motes discharge. thus even when no lightning is passing or rain falling, the air-earth current may be abnormally large under thunderclouds. recently c. fe. p. brooks, from a sum- mary of thunderstorms at different parts of the earth, has cal- culated that over the earth as a whole, about 1oo lightning strokes occur per second. if these were all of one sign, and each brought 20 coulombs to the earth, this would be equivalent to a vertical current of about 410778 amp./cm*. many lightning strokes pass, however, between clouds, and a large minority according to wilson bring down positive electricity. thus lightning cannot safely be treated any more than rain as a 261 negligible factor in the problem presented by the earth’s negative charge in fine weather. as to the other atmospheric electricity elements, in addition to the ions caught by the ebert apparatus which have values of the order 1-5 cm/sec. for their mobilities (7.e., their velocities under a potential gradient of 1 v/cm), there are ‘‘ heavy ”’ ions. these, having a mobility of only about sop of that of the light ions, are practically negligible so far as concerns the conductivity of the air and the air-earth current, unless their numbers much exceed those of the light ions. in or near large towns, judging by the large average number—16,000 per cm’—found in dublin by mcclelland and kennedy, heavy ions may play an important part. but in country districts, they seem no more numerous than light ions. thus the ordinary calculations of conductivity based on light ions alone should not be much in error. for the mean numbers mm #2 of light positive and negative ions per cm*, as observed at a number of land stations, chauveau gives 750 and 630 respectively. for the corresponding figures from sea observations—mostly by the surveying ship of the carnegie institution of washington, chauveau gives #,=730, t= 580. the areas represented are so comparatively few that we can only say that there is little if any difference between land and sea. if we take the value now accepted for the ionic charge 4-8x 10-19, we obtain from chauveau’s mean land values as the free ionic charges positive and negative per cubic metre. f036. 5.5.0, e2,=0-30 e.s.u. if k, and ks are the mobilities, the air-earth current, assuming the air without vertical velocity, is given by i=(k; nit+ke ne) ep where ¢ is the ionic charge, p the potential gradient per cm. this may be written i=ap, where a the conductivity is given by in = ar + ae with at = kinyenae = kenee. x and dz the so-called conductivities for positive and negative ions are usually measured separately. if we suppose, as an example, = %2= 500, and h\=ke=1-5 v/em=450 e.s.u., we have a=a,-+-as=2x 500x450 4:8 xk 1o-m%=2:2k 10-7? = e.s.u. there are only a few stations where regular observations are made of x, and these are mostly at only one or two hours of the day. the following mean yearly values have been obtained for x and 7 a(e.s.u.) — ifamp/em?) davos: hrly. obs. . i909-io lyr. 2:68 i-71 potsdam: frly. obs. . i910-i =lyr. o-g5xio4 2-37x107' tortosa: obs. near 11 h.. . 1918-23 6 yrs. 1-80 2-03 val joyeux: —. ; obs. at gh., 13h., 17h. 1924 tyr. i-43 1-57 speaking generally, the diurnal and annual variations of a are in the opposite direction to those of potential gradient. at all the stations mentioned above x is largest in summer, and at potsdam and davos it is largest in the early morning, when p is least. the vertical fine weather current would seem to be a less variable clement than the others. the ionisation of the atmosphere has several possible sources, including y rays from radium and thorium in the ground, radio- active emanation in the air, photo-electric effects at the ground, ultra-violet light especially at higher levels, and penetrating radiation. the last-mentioned source has been demonstrated by experiments with closed vessels. near the earth’s surface it is less over sea than over land, but still sensible. over land, as we ascend, it diminishes at first according to the observations of hess and kothorster, but after attaining a minimum at a height of 500 metres or a little over, it increases again, attaining much higher values than near ground level. more recent observations made by r. a. millikan confirm this increase, though making it considerably less than kilhorster supposed. millikan has found the radiation to include rays of different penetrating power, some much more penetrating than the hardest y rays. the source of this radiation is still problematical. 262 for references the reader ts referred to the treatises by chauveau and mathias described below. chauveau's vol. 1 is entirely his- torical. the volume edited by mathias has an elaborate bibliography at the end of each chapter, as well as a very full name index. l. a. bauer, terrestrial afagnetism, vol. 29, p. 23, etc.; b. chauveau, electricite atmospherique, in 3 vol.; g. c. simpson, british antarctic expedition 1910-3 meteorology, vol. 1, p. 312; e. mathias 7traite delectricite atmospherigue et tellurique; g. c. simpson, phil. trans. a. vol. 209, p. 405; c. t. r. wilson, phil. trans. a. vol. we pe 7% (cuc i.) atom (see 2.870), structural units —through the experi- mental discoveries of the second half of the roth century it became gradually clear that the atoms of the elements, far from being indivisible entities, had to be. thought of as aggregates built up of separate particles. thus from experiments on elec- trical discharges in rarified gases and especially from a closer study of the so-called cathode rays, one was led to recognise the existence of small negatively charged particles the mass of which was found to be about 2,000 times as small as the mass of the lightest atom, the hydrogen atom. these small particles, which may be regarded as atoms of negative electricity are now, fol- lowing johnstone stoney, generally called electrons. through the investigations of j. j. thomson and others convincing evi- dence was obtained that these electrons are a constituent of every atom. on this basis a number of the general properties of matter, especially as regards the interaction between matter and radiation, receive a probable explanation. in fact the assumption that electrons are vibrating around positions of stable equilibrium in the atom offered a simple pic- ture of the origin of spectral lines which allowed the phenomena of selective absorption and dispersion to be accounted for in a natural way. even the characteristic effect of magnetic fields on spectral lines discovered by zeeman could, as was shown by lorentz, be simply understood on this assumption. the origin of the forces which kept the electrons in their positions remained for a time unknown, as well as the way in which the positive electrification was distributed within the atom. from experi- ments on the passage through matter of the high speed particles expelled from radioactive substances, however, rutherford was in rgri jed to the so-called nuclear model of the atom. accord- ing to this the positive electricity is concentrated within a nucleus of dimensions very small compared with the total space occupied by an atom. this nucleus is also responsible for practically the whole of the atomic mass. true properties of the elements.—the nuclear theory of the atom has afforded a new insight into the origin of the properties of the elements. these properties can be divided into two sharply distinguished classes. to the first class belong most of the ordinary physical and chemical properties. these depend on the constitution of the electron cluster round the nucleus and on the way in which it is influenced by external agencies. this, however, will depend on the attractive force due to the nucleus which keeps the cluster together. on account of the small size of the nucleus compared with the distance apart of the electrons in the cluster, this force will to a high approximation be deter- mined solely by the total electric charge of the nucleus. the mass of the nucleus and the way in which the charges and masses are distributed among the particles making up the nucleus itself will only have an exceedingly small influence on the behaviour of the electronic cluster. to the second class belong such properties as the radioactivity of the substance. these are determined by the actual internal structure of the nucleus. in the radioactive processes we wit- ness, in fact, explosions of the nucleus in which positive or nega- tive particles, the so-called a and @ particles, are expelled with very great velocities. the complete independence of the two classes of properties is most strikingly shown by the existence of substances which are indistinguishable from one another by any of the ordinary physical and chemical tests, but of which the atomic weights are not the same, and whose radioactive properties are completely different. any group of two or more such substances are called isotopes (q.v.), since they occupy the same position in the classification of the elements according toa ordinary physical and chemical properties. the first evidence of atom their existence was found in the work of soddy and other inves- tigators on the chemical properties of the radioactive elements. it has been shown that isotopes are found not only among the radioactive elements, but that many of the ordinary stable ele- ments consist of isotopes, for a large number of the latter that were previously supposed to consist of atoms all alike have been shown by aston’s investigations to be a mixture of isotopes with different atomic weights. moreover the atomic weights of these isotopes are whole numbers, and it is because the so- called chemically pure substances are really mixtures of iso- topes, that the atomic weights are not integers. inner structure —the inner structure of the nucleus is still but little understood, although a method of attack is afforded by rutherford’s experiments on the disintegration of atomic nuclei by bombardment with a particles. indeed, these experi- ments may be said to have started a new epoch in natural philosophy in that for the first time the artificial transformation of one element into another has been accomplished (see trans- mutation of elements). in what follows, however, we shall confine ourselves to a consideration of the ordinary physical and chemical properties of the elements and the attempts which have been made to explain them on the basis of the concepts just outlined. tue relationships between the elements periodicity of elements.——it was recognised by mendelejeff that when the elements are arranged in an order which is prac- tically that of their atomic weights, their chemical and physical properties show a pronounced periodicity. a diagrammatic representation of this so-called periodic table is given in table i., which represents in a slightly modified form an arrangement first proposed by julius thomsen. in the table the elements are denoted by their usual chemical symbols, and the different ver- tical columns indicate the so-called periods. the elements in successive columns which possess homologous chemical and phys- ical properties are connected by lines. the meaning of the square brackets around certain series of elements in the later periods, the properties of which exhibit typical deviations from the simple periodicity in the first periods, will be mentioned below. radiation.—the discovery of the relationship between the elements was primarily based on a study of their chemical prop- table i, 19 k—37 r 9 cu-—47 ag ‘9 f—17 c1830 zn—48 cd 0 ne—18 a-{131 ga—49 in "32 ge—50 sn 833 as—51 sb 134 se—52 te "35 br—53 i 136 kr—54 x atom erties. later it was recognised that this relationship appears also very clearly in the constitution of the radiation which the ele- ments emit or absorb in suitable circumstances. in 1883 balmer showed that the spectrum of hydrogen, the first element in the table, could be expressed by an extremely simple mathematical law. this so-called balmer formula states that the frequencies v of the lines in the spectrum are given toa close approximation by i i . (are a (1) where r is a constant, and where #’ and #’’ are whole numbers. if x” is put equal to 2 and »’ is given successively the values 3,4, . . . the formula gives the frequencies of the series of lines in the visible part of the hydrogen spectrum. if »” is put equal to 1 and #’ equal to 2, 3, 4, . . . a series of ultra-violet lines is obtained which was discovered by lyman in 1914. to 7” =3, 4, . . . correspond series of infra-red hydrogen lines which also have been observed. rydberg in his famous investigation of line spectra more than 30 years ago was able to analyse in a similar way many spectra of other elements. just as in the case of hydrogen he found that the frequencies of a line-spectrum (such as that of sodium) could be represented by a formula of the type yea" = t’ (2) where t”’, t’ can be approximately represented by =a (3) ax is a constant for any one series, but takes different values a1, 42... for the different series, while 1 takes a set of succes- sive integral values. r is constant throughout for all spectra, and is the same constant as that appearing in (1); it is generally called the ‘‘ rydberg number.”’ in many spectra the terms of most series are multiple, i.e., the terms which we consider as forming a series do actually form two, three or more series cor- responding to two, three or more slightly different values of ax. rydberg also discovered that the spectra of elements occupying homologous positions in the periodic table were very similar to each other, a similarity which is especially peepounced as regards the multiplicity of the terms. moseley’s discovery.—the study of x-ray eneeea made pos- sible by the work of laue and bragg brought out relations of a still simpler kind between different elements. thus moseley (g.v.) in 1913 made the fundamental discovery that the x-ray spectra of all elements show a striking similarity in their struc- ture, and that the frequencies of corresponding lines depend in a very simple way on the ordinal number of the element in the periodic table. moreover the structure of these spectra was very like that of the hydrogen spectrum. the frequency of one of the strongest x-rav lines for the various elements could for instance be given approximately by y=n?r (<-3 (4) (5) where r is again the rydberg constant and n the ordinal num- ber of the element in the periodic table. the extreme simplicity of these formulae enabled moseley to settle any previous uncertainty as to the order of the elements in the periodic table, and also to state definitely the empty places in the table to be filled up by elements not yet discovered. atomic numibers.—in the nuclear model of the atom, the ordinal number of an element in the periodic table receives an extraordinarily simple interpretation. in fact, if the numerical value of the charge on an electron is taken as unity, this ordinal number, which is often called the “‘ atomic number ”’ (g.v.), can simply be identified with the magnitude of the nuclear charge. this law which was foreshadowed by j. j. thomson’s investi- gations of the number of electrons in the atom as well as by rutherford’s original estimate of the charge on the atomic nucleus, was first suggested by van den broek. it has since been estab- 263 lished by refined measurements of the nuclear charge, and it has proved itself an unerring guide in the study of the relation- ship between the physical and chemical properties of the cle- ments. this law also offers an immediate explanation of the simple rules governing the changes in the chemical properties of radioactive elements following the expulsion of a or 8 particles. the quantum theory the discovery of the electron and of the nucleus was based on experiments, the interpretation of which rested on applica- tions of the classical laws of electrodynamics. as soon, how- ever, as an attempt is made to apply these laws to the interaction of the particles within the atom, in order to account for the phys- ical and chemical properties of the elements, we are confronted with serious difficulties. consider the case of an atom containing one electron: it is evident that an electrodynamical system con- sisting of a positive nucleus and a single electron will not exhibit the peculiar stability of an actual atom. even if the electron might be assumed to describe an elliptical orbit with the nucleus in one of the foci, there would be nothing to fix the dimensions of the orbit, so that the magnitude of the atom would be an un- determined quantity. moreover, according to the classical theory the revolving electron would continually radiate energy in the form of electromagnetic waves of changing frequency and theelec- tron would finally fall into the nucleus. in short, all the promising results of the classical electronic theory of matter would seem | at first sight to have become illusory. it has nevertheless been possible to develop a coherent atomic theory based on this pic- ture of the atom by the introduction of the concepts which formed the basis of the famous theory of temperature radiation developed by planck in rgoo. this theory marked a complete departure from the ideas which had hitherto been applied to the explanation of natural phenomena, in that it ascribed to the atomic processes a certain element of discontinuity of a kind quite foreign to the laws of classical physics. one of its outstanding features is the appear- ance in the formulation of physical laws of a new universal con- stant, the so-called planck’s constant, which has the dimen- sions of energy multiplicd by time, and which is often called the ‘elementary quantum of action.” we shall not enter upon the form which the quantum theory exhibited in planck’s original investigations, or on the important theories developed by ein- stein in 1905, in which the fertility of planck’s ideas in explaining various physical phenomena was shown in an ingenious way. we shall proceed at once to explain the form in which it has been possible to apply the quantum theory to the problem of atomic constitution. this rests upon the followmg two postulates:— 1. an atomic system is stable only in a certain set of states, the ‘' stationary states,’ which in general corresponds to a dis- crete sequence of values of the energy of the atom. every change in this energy is associated with a complete “ transition ” of the atom from one stationary state to another. 2. the power of the atom to absorb and emit radiation is governed by the law that the radiation associated with a transi- tion must be monochromatic and of frequency pv such that hy= e,— e2 (6) where / is planck’s constant and f, and eg, are the energies in the two stationary states concerned. the first of these postulates aims at a definition of the in- herent stability of atomic structures, manifested so clearly in a great number of chemical and physical phenomena. the second postulate, which is closely related to einstein’s law of the photo- electric effect, offers a basis for the interpretation of line spec- tra; it explains directly the fundamental spectral law expressed by relation (2). we see in fact that the spectral terms appearing in this relation can be identified with the energy values of the stationary states divided by #. this view of the origin of spectra has been found to agree with the experimental results obtained in the excitation of radiation. this is shown especially in the dis- covery of franck and hertz relating to impacts between free 264 electrons and atoms. they found that an energy transfer from the electron to the atom can take place only in amounts which correspond with the energy differences of the stationary states as computed from the spectral terms. the hydrogen spectrum.—from the balmer formula (1) and the quantum theory postulates, it follows that the hydrogen atom has a single sequence of stationary states, the numerical value of the energy in the #'+ state being r//#?. applying this result to the nuclear model of the hydrogen atom, we may as- sume that this expression represents the work necessary to remove the electron from the nth state to an infinite distance from the nucleus. if the interaction of the atomic particles is to be explained upon the laws of classical mechanics, the electron in any one of the stationary states must move in an elliptical orbit about the nucleus as focus, with a major axis whose length is proportional to #*. the state for which # is equal to z may be considered as the normal state of the atom, the energy then being aminimum. for this state the major axis is found to be approxi- mately 10-8 centimetres. it is satisfactory that this is of the same order of magnitude as the atomic dimensions derived from experiments of various kinds. it 1s clear, however, from the nature of the postulates, that such a mechanical picture of the stationary states can have only a symbolic character. this is, perhaps most clearly manifested by the fact that the frequencies of the orbital revolution in these pictures have no direct connec- tion with the frequencies of the radiation emitted by the atom. nevertheless, the attempts at visualising the stationary states by mechanical pictures have brought to light a far-reaching analogy between the quantum theory and the classical theory. this analogy was traced by examining the radiation processes in the limit where successive stationary states differ compara- tively little from each other. here it was found that the fre- quencies associated with the transition from any state to the next succeeding one tend to coincide with the frequencies of revolution in these states, if the rydberg constant appearing in the balmer formula (1) is given by the following expression: anctin r=—, (7) where e and m are the charge and mass of the electron and / is planck’s constant. this relation is actually found to be fulfilled within the limits of the experimental errors involved in the measurements of e, m and #, and may be considered to establish a definite relation between the spectrum and the atomic model of hydrogen. correspondence principle.—the considerations just mentioned constitute an example of the application of the so-called “ corre- spondence principle’ which has played an important part in the development of the theory. this principle gives expression to the endeavour, in the laws of the atom, to trace the analogy with classical electrodynamics as far as the peculiar character of the quantum theory postulates permits. on this line much work has been done in the last few years, and quite recently in the hands of heisenberg has resulted in the formulation of a rational quantum kinematics and mechanics. in this theory the concepts of the classical theories are from the outset tran- scribed in a way appropriate to the fundamental postulates and every direct reference to mechanical pictures is discarded. heisenberg’s theory constitutes a bold departure from the classical way of describing natural phenomena but may count as a merit that it deals only with quantitics open to direct observa- tion. this theory has already given rise to various interesting and important results, and it has in particular allowed the balmer formula to be derived without any arbitrary assump- tions as to the nature of the stationary states. however, the methods of quantum mechanics have not yet been applied to the problem of the constitution of atoms containing several electrons, and in what follows we are reduced to a discussion of results which have been derived by using mechanical pictures of the stationary states. although in this way a rigorous quantita- tive treatment is not obtainable it has nevertheless been possi- ble, with the guidance of the correspondence principle, to obtain a general insight into the problem of atomic constitution. atom the spectra of elements of higher atomic number the hydrogen spectrum may be considered as evidence of a step-like process in which an electron is captured and bound increasingly strongly in the field surrounding the nucieus, the stages of this process being the stationary states of the atom. simple arguments lead to the conclusion that the stages corre- sponding to the binding of an electron by a nucleus of any given charge will be represented by a similar sequence of stationary states and that the energy w, necessary to remove the electron from the m*» state will be given by the expression: w,= n? a (8) n* where n is the atomic number of the elements under considera- tion. these states may be visualised as mechanical orbits of the electron in which the major axis is n times as small as the major axis in the corresponding orbit in the hydrogen atom. the spectrum associated with the binding process under considera- tion is represented by the formula: ats i i r= rcc gr) o for n=2, this tormula actually represents the spectrum which is emitted by a singly ionised helium atom, 7.e., a helium atom, which has lost one of its electrons. spectra of this type have not yet been observed for values of n larger than 2, but it will be seen that formula (9) includes the approximate formulae (4) and (5) representing the frequencies of the strongest lines in the x-ray spectra of the elements. this may be understood if we assume that an x-ray spectrum is associated which changes in the state of binding of one of the electrons in the inner region of the atom, where, at least when the atomic number is large, the force on the electron due to the nucleus will far outweigh the forces due to the other electrons, and where consequently the presence of these electrons will have a comparatively small influence on the strength of the binding. influence of electrons—in general the mutual influence of the electrons is very considerable. consider the stages by which an electron is captured by an atom of which the nucleus already has s electrons circulating round it. in the initial stages of this process while the orbits may be supposed to have dimensions which are large compared with the orbital dimensions of the electrons previously bound, the repulsive forces from these latter electrons may be assumed to neutralise s units of the nuclear charge, and the resultant force will be approximately the same as when an electron is circulating round a nucleus of atomic number n-s. in the later stages, when the dimensions of the orbit of the new electron arc smaller, the other electrons can no longer be considered to act as a single central charge, and their repulsion cannot be easily determined. thus the con- ditions become more complicated, and the stationary states can no longer be treated by picturing the motion of the new electron as following a keplerian ellipse. it has been found, however, that many features of the resulting spectra would be explained by assuming the added electron to move in a plane central or- bit consisting of a sequence of quasi-elliptic loops. in contrast to a keplerian orbit, however, the single loops are not closed but the successive maximum radii will be placed at constant angular intervals on a circle with the nucleus at the centre. for such central orbits it is possible, as was first shown by sommer- feld, to select from the continuous multitude of possible orbits a set of orbits which mav be taken as representing stationary states in the sense of the quantum theory. these states are labelled with two integral numbers; the one, denoted by 2, corresponds to the integer appearing in the balmer formula and is called the principal quantum number. the other, denoted by k, may be called the subordinate quantum number. for any given value of #, the number & can take the values 1, 2, 3... #, corre- sponding to a set of orbits with increasing minimum distance from the nucleus. for a given value of k increasing values of # correspond to orbits which exhibit an increasing maximum distance from the nucleus, but which are similar in size and shape in the region where the electron comes nearest to the nucleus. atom for the work necessary to remove an electron in an #, orbit completely from the nucleus, the theory leads to the following approximate expression : » ra wr2=(n—s) =a (10) where a, depends only on the subordinate quantum number &, and approaches zero for increasing &. if s is equal to n-1, we see that the w,,z when divided by & co- incide exactly with rydberg’s expressions (3) for the spectral terms of the ordinary series spectra of the elements. these spectra may therefore be considered as evidence of processes, represent- ing the last stage in the formation of a neutral atom, in which a nucleus of charge ne, which holds already n-1 electrons bound in its field, is capturing an nt electron. in recent years it has been found that many elements under suitable conditions be- sides their ordinary spectra also emit spectra for which the terms can be represented by r (92 — a)? (i) where # may take the integral values 2, 3, 4. . . . comparing (11) with formula (10) we see that these spectra must be ascribed to atoms, which after having lest p electrons are rebinding an electron in the field of the remaining atomic ion. this interpretation of series spectra allows also the rules gov- erning the possible combinations of spectral terms to be ex- plained. in fact, it has been found that only those lines appear in the spectrum for which the k-values of the spectral terms involved differ by one unit. from an investigation of the con- stitution of the radiation which on classical electrodynamics would be emitted from an electron performing a central motion, this rule can be shown to be a simple consequence of the corre- spondence principle. multiplet structure —the multiplet structure exhibited by the terms of most series spectra makes it necessary to assume that the motion of the electron involved in the emission of these spectra is somewhat more complicated than the simple central motion described above. an analysis based on the correspon- dence principle indicates that this motion may be described as a central motion on which is superposed a uniform precession of the orbital plane round an invariable axis in space. fora time, however, it seemed very difficult to obtain any closer connection between the observed structures and the above hypothesis of the constitution of the atom. in particular the remarkable analogy between the finer structures of the optical spectra the x-ray spectra, which had been brought out by the experi- ments, was very puzzling. the study of the strange anomalies exhibited by the effect of a magnetic field on the components of the optical multiplets has, however, quite recently led to the view that the electron itself carries besides its electric charge, also a magnetic moment which may be associated with a swift rotation round an axis through its centre. this new assumption allows not only the anomalous zeeman efiect to be accounted for, but affords at the same time a natural explanation for the empirical rules governing the dependency of the widths of the multiplet structures on the atomic number. t =p? atomic constitution and the periodic table soon after the discovery of the electron it was recognised that the relationships between the physical and chemical properties of the elements expressed in the periodic table point towards a group-structure of the electronic distribution in the atom. fun- damental! work on these lines was done by j. j. thomson in 1904. after the discovery of the nucleus and the simple interpretation of the atomic number given above, his work has been followed up with great success especially by kossel and lewis. valency properties —it is suggested that the electrons within the atom possess a tendency to form stable groups, each con- taining a definite number of electrons, which in the neutral state of the atom surround the centre of the atom like successive shells or layers. an explanation of the simple valency propertics holding for the second and third period of the periodic table was 265 for instance obtained by assuming that there was a tendency to form completed shells each containing eight electrons. the single valency of sodium and the double valency of magnesium are ascribed to the facility with which the neutral atoms of these elements can loose one or two electrons respectively, as the atomic ions remaining would then contain completed shells only. on the other hand the double negative valency of sulphur and the single negative valency of chlorine are ascribed to the tendency of their outermost shells to take up two or one addi- tional electron respectively in order to form a complete shell of eight electrons, like that contained in the neutral atom of the inactive gas argon. statical arrangement of electrons —attempts have been made to associate the existence of such groups with statical configura- tions of electrons possessing a high degree of symmetry. the pres- ence of groups of eight electrons for instance has been explained as an arrangement of electrons at the corners of a cube. however suggestive these ideas have been in affording pictures of the constitution of chemical compounds, they do not allow a direct connection with other properties of the atom to be established; the main difficulty being that stable statical arrangements of the electrons are incompatible with the nuclear theory of the atom. in the meantime, however, it has been possible to con- nect the group structure of the electronic cluster in the atom with the quantum-theory interpretation of spectra. thus the constitution of the neutral atom in its normal state can be in- vestigated by imagining a process by which n electrons one after one are captured and bound in the field of force surrounding a nucleus of charge ne. | to each step there corresponds a multitude of stages, 1.e., stationary states, in which the electron is more and more firmly bound to the atom. the final state, in which binding is strongest, corresponds to the normal state of the atomic ion. a definite connection between the spectra and the group structure was now established by assuming that in the normal atom only a limited number of electrons can be bound in states visualised as orbits characterised by definite values of the quantum numbers nandk. the electrons bound in orbits corresponding to a given value of # are said to form an n-quantum group, which in its finally completed stage will contain # subgroups, corresponding to the possible values 1, 2... # which & may take. for a sufficiently large nuclear charge, the strength with which the electrons in the different subgroups belonging to one and the same group are bound will be nearly equal. in the gradual build- ing up of the groups in atoms with increasing nuclear charge, it is, however, to be noted that when an », orbit appears for the first time in the neutral atom, the strength of the binding will depend very considerably on the value of &. this is due to the circumstance that this quantum number fixes the closest distance to which the electron may approach the nucleus. the screening of the nuclear charge by the other electrons in the atom may therefore be very different for orbits corresponding to different values of k, and the effect on the strength of the binding can be so large that an orbit characterised by certain values of 7 and & may correspond to a stronger binding than an orbit for which 7 is smaller but & larger. this offers a natural explanation of one feature of the periodic table, namely that the periods grow gradually larger, while there appear se- quences of elements which differ comparatively little in their chemical and physical properties. such a sequence marks a stage in the development of an #2-quantum group, which consists in the addition of a subgroup corresponding to a value of k which was previously not yet represented in that group, and which takes place after the building up of a group corresponding to a higher value of # has already begun. in fact, during the addition of the subgroup a temporary standstill will occur in the development of the latter group, the constitution of which will primarily determine the chemical affinity of the atom, since 1t contains the most loosely bound electrons. in the accompanying table (table ii.) is given a summary of the structure of the normal state of the neutral atoms of the clements. the figures before the different elements are the 266 atom table ii. pr mm i tt rr i cm 1 h i 2 he 2 | | 2 i 4 be 2 2 ~ 5 b e 2 i | . 1d ne 2 2 6 | | 11 na 2 2 6 i 12 mg 2 2 6 2 | 13 ae 2 2 6 2 i 18 a 2 2 6 2 6 | | iogk 2 2 6 2 6 i 20 ca 2 2 6 2 6 2 21 se 2 2 6 2 6 i 2 22°) 2 2 6 2 6 2 2 29 cu 2 2 6 2 6 10 i 7 30 zn 2 2 6 2 6 10 2 31 ga 2 2 6 2 6 i0 2 i 36 kr 2 2 6 2 6 i0 2 6 37 rb 2 2 6 2 6 io 2 6 i 38 sr 2 2 6 2 6 io 2 6 2 39 y 2 2 6 2 6 i0 2 6 i 2 40 zr 2 2 6 2 6 i0 2 6 2 2 47 ag 2 2 6 2 6 i0 2 6 10 i | 48 cd 2 2 6 2 6 i0 2 6 10 2 49 in 2 2 6 2 6 io 2 6 10 2 i 64° x 2 2 6 2 6 10 2 6 10 2 6 55 cs 2 2 6 2 6 10 2 6 10 2 6 i ¢ 56 ba 2 2 6 2 6 io 2 6 10 2 6 2 57 la 2 2 6 2 6 io 2 6 10 2 gs: ¥{ 2 58 ce 2 2 6 2 6 10 2 6 10 i 2 6 i 2 59 pr 2 2 6 2 6 i0 2 6-10. «2 2 6 i 2 71 g 2 2 6 2 6 x0 2 6 10 14 2 6 1 2 72h 2 2 6 2 6 io gio: tq 2 6 2 2 79 au 2 2 6 2 6 i0 2 6 10 i4 2 6 10 i 80 hg 2 2 6 2 6 i0 2 6 10 1i4 2 6 10 2 81 ti 2 2 6 2 6 10 2 6 10 i4 2 6 10 2 : 86 em 2 2 6 2 6 i0 2 6 10 i4 2 6 10 2 6 87 — 2 2 6 2 6 10 2 6 10 i4 2 6 10 2 6 i 88 ra 2 2 6 2 6 io 2 6 10 i4 2 6 10 2 6 2 89 ac 2 2 6 2 6 10 2 6 10 i4 2 6 10 2 6 2 i atomic numbers, which give the total number of electrons in the neutral atom. the figures in the different columns give number of electrons in orbits corresponding to values of the principal and subordinate quantum numbers standing at the top. a comparison with the periodic table (table i.) will show that those elements, which in chemical respect are homologous will have the same number of electrons in the electronic groups most loosely bound, containing the so-called valence-electrons. the atoms of elements which in table i. are enclosed in brackets possess electronic configurations in which a subgroup is being added to a group, whose principal number is less than the group containing the typical valence-electrons. an especially conspic- uous example of such a completion of an inner group is offered by the elements forming the family of the rare earths. here we witness the addition of the fourth subgroup to the 4-quantum group, which begins first in ce (58) while the addition of the third subgroup was already finished in ag (47). table il. is in general agreement not only with the optical spectral evidence but also with that in the region of x-rays. as mentioned earlier, we sec in x-ray spectra a change in the binding of an electron in the interior of the atom. this takes place when, for instance, by the impact of a swiftly moving particle on the atom, an electron is removed from one of the electronic groups, and its place is taken by an electron belonging to a group for which the binding energy is smaller. as an ex- ample it may be stated that the strong x-ray whose frequency is approximately represented by formula (4) is emitted when an electron has been removed from the 1-quantum group, and one of the 22 electrons performs a transition so as to occupy the empty place. the line represented approximately by formula (5) originates from a transition by which a 33 electron takes the place left open upon the removal of a 22 electron. the question how many electrons there are in the various groups and subgroups has been subject to much discussion in the last few years. table i. is the temporary result of this dis- cussion and seems to give an adequate description of the spectral as well as the chemical evidence. it is clear that a full theoretical treatment of the problem cannot be obtained from considerations based only on the simple picture of central orbits. such a treatment will essentially involve an examination of those features of the binding of the electrons, which appear in the multiplet structure of spectral lines. indeed it 1s very probable that the idea that the electron itse)f has magnetic properties may give the clue to the interpretation of the empirical rules governing the number of electrons in the group structure of the atom. in this article we have tried to give an idea of the fertility of the use of mechanical pictures visualising stationary states in atomic energy—atomic weights the analysis of the properties of the elements. notwithstanding the valuable suggestiveness of such an hypothesis it must once more be emphasised that such pictures do not allow the atomic phenomena to be interpreted quite rationally within the frame of the postulates of the quantum theory. for the adequate description of the properties of atoms it seems impossible to rely directly upon any concept of classical electrodynamics. on the contrary it appears imperative that any such concept should from the outset be transcribed in accordance with the methods of the new quantum mechanics, alluded to above. (see also atomic weights; chemistry; gases, electrical properties of; isotopes; quantum theory.) brstriograpny.—e. n. de c. andrade, the structure of the atom; g. birtwistle, the quantum theory of the atom; n. bohr, the theory of spectra and atomic constitution; 1. d. main smith, chemistry and atomtc structure; a. sommerfield, atomic structure and spectral lines. (n. bo.) atomic energy.—by this expression is generally meant energy associated with the inner nuclei of atoms, in contra- distinction to energy of translation or thermal agitation pos- sessed by the atoms moving as units, or chemical energy which is associated with their outer systems of electrons. soon after the discovery of radioactive elements it was demonstrated that their radiations were entirely unaffected by temperature or chem- ical combination, and that therefore they must be supplied by a source of energy more deeply seated than any hitherto suspected. on rutherford’s theory of the atom, this source could only be the nucleus itself which in the process of spontaneous disinte- gration liberated energy in the form of radiations. measurements showed that one gramme of radium gave out heat at the rate of 1oo-gramme calories per hour, and would continue to do so ata hardly diminished rate for many centuries. this radioactive energy was, atom for atom, so vastly greater than that liberated in the most violent chemical reactions that it aroused great interest, and was hailed as the source of the sun’s heat. misled by the idea that radioactivity could be “induced” in otherwise non-radioactive elements, radioactive energy was eagerly seized by speculative writers (see h. g. wells, the world set free, 1914) as the energy of the future. further investigation failed, however, to support these claims. it was calculated that even if the sun were composed entirely of radioactive matter, the energy produced would still be quite inadequate to meet the demands of science. experiments supposed to prove the exist- ence of “induced ”’ radioactivity were found to bear in reality a different interpretation and the excessive rarity of the radio elements removed any hope of using these as a source of terrestrial energy on a practical scale. with the coming of relativity (see relativity) and the dis- covery of isotopes (see isotopes) the matter took a new aspcct. the whole number rule removed the last obstacle in the way of the electrical theory of matter, that all atoms are composed of protons and electrons, the atoms of positive and negative electricity. according to rutherford’s nucleus atom theory, in ' the atom of a normal element all the protons and about half the electrons are packed together to form a central positively charged nucleus, which is surrounded by the remaining electrons. it can be shown that if we bring two charges of opposite sign as close together as they are in the nucleus, their ficlds will affect each other in such a way that the mass of the system will be reduced. this reduction is called the packing effect. in the atom of hy- drogen with a nucleus of a single proton there can be no packing efiect, so that it will be abnormally heavy. measurements by means of the mass-spectrograph demonstrate conclusively that the mass of a hydrogen atom, consisting of one proton and one electron, is that accepted by chemists, namely 1-0077, whereas that of the helium atom, consisting of a nucleus of four protons and two electrons and two exterior electrons, is 4-00. hence, whatever the explanation, it is certain that if it were possible to transmute hydrogen into helium, mass would be lost, and there- fore, by the theory of relativity, energy liberated. on the latter theory, mass and energy are interchangeable, and the energy associated with a mass m is mc? where c is the velocity of light. 267 for quantities of matter in ordinary experience this quantity of energy is prodigious. ‘take the case of one gramme atom of hydrogen, that is to say, the quantity of hydrogen in 9 cu.-cm. of water. if this is entirely transformed into helium the energy liberated will be -0077 x9 x 10° = 6-93 x10! ergs. expressed in terms of heat this is 1-66 x 10" calories or in terms of work 200,000 kilowatt hours. within a tumbler of water hes sufficient energy to propel the ““maurctania” across the atlantic and back at full speed. here we have a supply equal even to the demands of astronomers. eddington remarks that if only ten per cent of the hydrogen in the sun were transformed into helium, enough energy would be liberated to maintain its present radiation for a thousand million years. there can be little doubt that the vast energy of the stars is kept up by the loss of an insignificant fraction of their mass. whether this process is a degradation of hydrogen, or simple annihilation of matter by the coalescence of protons and electrons, is unknown. how long it will be before man can release and control this energy, and to what uses he will put such vast potentialities, are subjects for the philosopher. the first step has already been taken, for sir ernest rutherford has succeeded in causing transmutation in several elements, only, of course, in inconceivably small quan- tities, by bombardment with swift alpha rays. if scientific knowledge maintains its present rate of progress, the balance of probability is in favour of ultimate success, but this appears so far off that almost any speculation is permissible. it may be that the operation, once started, is uncontrollable and that the new stars which flare out from time to time are but the notifica- tion of successful large-scale experiments on far distant worlds. it may be that the highest form of life on our planet will one day discover supreme material power, or cataclysmic annihila- tion, in the same ocean wherein, we are told, its lowest forms originally evolved. brbliography.—phases of modern science, a collective work (1925); ‘‘ atomic theory and mechanics,” niels bohr, suppl. to nature (dec. 5 1925). (f. w. a.) atomic weights.—atomic weights have been defined as “ the relative weights of the atoms of chemical elements referred to a common standard.” this statement still serves as the sim- plest indication of the fundamental idea involved, although it now needs amplification. the concrete development of the idea was first effected in 1803 by john dalton, an english chemist, when he converted the vague atomistic theory of the ancient grecks into a highly valuable scientific asset by means of the concept of atomic weights. the chemical atomic theory thus initiated has been strengthened by modern investigation, and is to-day entrenched in a well-nigh impregnable position. | practical and scientific interest—atomic weights are quan- tities of great practical and theoretical importance. they record the operation of the chemical law of definite combining propor- tions; hence they are the basis of quantitative chemical analysis, and are in everyday use throughout the world. because of the parallelism between gravitational effect and inertia, they record also the relative masses of the atoms of the elements. they possess an extraordinary degree of definiteness, since the law of combining proportions is one of the few known precise laws of the universe. far deeper in meaning than the accidental astronom- ical “ constants,” such as the length of the day or the length of the year, the atomic weights of the simple elements and of the individual isotopes (see isotopes) stand out as among the peculiar and basic attributes of those 92 elementary substances of which everything is composed. their interpretation is closely concerned with our inferences concerning the nature of things. dalton’s views.—simple as the original concept of atomic weights seems to be, it nevertheless presents problems which are rather complex. for example, 22-997 grammes of sodium com- bine with 126-932 grammes of iodine to form sodium iodide. this ratio of the combining weights of these elements appears to be invariable. as dalton pointed out, these weights must depend on the relative weights of the respective atoms; no other simple explanation is conceivable. there is in the expcrimental result, 268 however, nothing which shows whether the sodium and iodine combine atom for atom, or whether one atom of sodium com- bines (for example) with two of iodine. dalton himself per- ceived that this latter happening might in many cases occur; indeed it is the essence of his law of multiple proportions. there is now every reason to believe that in this particular case of sodium and iodine the atoms actually combine one to one, and that the numbers given above represent really the relative weights of the atoms of sodium and iodine; but there are many less simple cases. for instance, 126-932 grammes of iodine combine with 20-035 grammes of calcium; here the latter number represents only half the atomic weight of calcium; because every molecule of calcium iodide is believed on excellent evidence to contain two atoms of iodine for every atom of calcium. sucha decision was beyond the reach of dalton. it is based chiefly upon three subsequent discoveries to be briefly described. _ avogadro’s h ypothesis.—in 1811 count amedeo avogadro di quaregna advanced the hypothesis, based upon gay lussac’s law of volumes, that equal volumes of gases under like condi- tions of temperature and pressure contain the same number of molecules, a molecule being defined as consisting usually of two or more atoms. this hypothesis (which has since been so amply confirmed as to become, in many minds, a statement of fact) furnishes the most important means of deciding between the multiples or sub-multiples of the combining proportions which are to be taken as the atomic weights, because it fixes the molec- ular weights and formulas of volatile clements and compounds (see chemistry). | — dulong and petit’s constant.—the second important means of deciding between possible multiples and sub-multiples of atomic weights was the discovery of dulong and petit (1818) that the atomic weight of an clement is about equal to a constant num- ber (6-3) divided by the specific heat. to be sure, this rule is not exact; but its inexactness is not usually great enough to affect it in its office of deciding the multiple or sub-multiple of the chemical combining proportion to be taken as the atomic weight. for example, the specific heat of calcium is about o-16; therefore its atomic weight is shown to be about 39-4, whereas the exact value found by chemical means is 40-07. crystalline similarity—a third method of answering the question exists in the similarity of the crystal forms of similar salts of allied elements, discovered by e. mitscherlich in 1821. if the atomic weight of one clement entering into such isomor- phous crystals is unknown, that multiple of the combining pro- portion of this element which corresponds to the formula in- dicated by the known salt may be taken as the true atomic weight. the full significance and essential consistency of these three methods of solving dalton’s unsolved problem were not realised until 1858, when a table of atomic weights identical in principle with that used today was published by s. cannizzaro. previous doubts concerning the criteria just described had caused many chemists to reject wholly the term “ atomic weights,” and to call the arbitrarily selected multiples merely by some such name as “‘ proportion numbers ”’ or “ chemical equivalents.” but the numbers now used (as regards the multiples chosen) inevitably involve the atomic theory, hence the adjective ‘‘ atomic ”’ is fitting. ‘“‘ weight ”’ also is fitting, since the values are deter- mined by means of the gravitational balance. the term “ atomic mass’ applies consistently only when inertia is the basis of measurement. the term ‘* chemical equivalent ” is now used to signify the atomic weight divided by the valence. standard of atomic weights.—the choice of the standard of atomic weights has varied. dalton chose the smallest atomic weight, that of hydrogen, as his standard. berzelius temporarily selected oxygen= roo as the standard of his system. later the chemical world returned to dalton’s practice, especially because (according to early work) it was believed that the atomic weight of oxygen is nearly the whole number 16, if hydrogen is taken as 1. finally, after it had been shown by e. w. morley and others that the ratio of the atomic weights of oxygen and hydrogen is in fact 15-878 to 1, it was decided, by general consent, in 1905, to abandon the standard h=1-ooo, retaining the standard o= atomic weights 16-000. the decision was based upon convenience. the per- manent choice of o=15:878 would have changed by nearly one per cent almost every other accepted value, and would have caused much confusion in previous quantitative statements. besides, more atomic weights approach whole numbers when oxygen is taken as exactly 16-000 than when any other usual standard ts chosen. a more weighty reason lay in the fact that most of the values are experimentally determined by relation to oxygen, and are referred to hydrogen only through that element. hence any subsequent change in the accepted ratio h : o (one of the most difficult to determine of all such ratios) would affect all the atomic weights, if hydrogen were chosen as the standard substance. the present, unit of the system of atomic weights is therefore exactly the sixteenth part of the atomic weight of oxygen. the atomic weight of hydrogen thus becomes 1:0077. the choice, on the whole, was a wise one; it has been justified by modern research, and has proved to be peculiarly fortunate, because probably all atoms of oxygen are alike in weight (see isotopes). atomic numbers and actual weights—atomic weights are numbers; that is to say, they represent ratios and are therefore devoid of physical dimensions. they are, however, very different from the quantities designated by j. a. r. newlands and li. g. j. moseley as “ atomic numbers,” which record the serial order of the places in the periodic classification of the chemical elements. no immediate knowledge of the actual weights of individual atoms is afforded by ‘‘ atomic weights,” unless the number of atoms in a given gross weight of some clementary substance is known. various researches have shown that 16 grammes of oxygen contain about 606x107! atoms; hence a single atom of oxygen must weigh 0-000,000,000,000,000,000,~ 000,026,4 gramme. the actual weights of other kinds of atoms must be in due proportion. | experimental determination.—the exact values of the chemi- cal combining proportions which form the basis of the table of atomic weights are found only by experimental work. there- fore, before the table is given, the necessary experimental meth- ods may well be brietly described. the first and most generally useful method employed for the purpose has as its object the determination of the precise amount of one element which is necessary exactly to combine with a given amount of some other element of known atomic weight. the experimental technique is that of the most refined quantitative chemical analysis. early extensive and careful investigations of this kind were conducted by j. berzelius, c. de marignac, j. b. a. dumas, j. s. stas and many others. recently most of the work in this direction has been conducted in the united states (e. w. morley, w. a. noyes, i. w. richards, g. p. baxter and others), although euro- pean investigators (especially b. brauner and o. honigschmid) have made important contributions. experimental work of this kind naturally involves the ob- servance of a number of essential conditions. comparatively few compounds of any given element are fit to serve as a means of determining its atomic weight, for the reason that com- . paratively few substances may be prepared in a perfectly pure state. the choice of the compounds to be employed is in some ways the most crucial part of the whole process, for with some compounds no result worthy of consideration could be obtained, even using the greatest care possible. having chosen wisely, the experimenter must prepare the needful substances, whatever they may be, in a state of very great purity. ile must never forget that every precipitate carries down with it contaminating impurities absorbed or included by the substance as it separates from the solution. he must remem- ber always that no receptacle necessary to contain the sub- stance is free from the possibility of being attacked or dissolved, thus affecting the result. moreover, precipitates are never wholly insoluble; and most substances will volatilise if heated to an excessive temperature. these complicating circumstances combine often in unexpected ways to introduce impurity, and the experimenter must not only guard against these dangers, but must prove by adequate tests that no such complication has atomic weights occurred. moreover, above all, he must not forget that oxygen, nitrogen and water are almost omnipresent; and continual care must be exercised lest in some way one of these impurities may affect the substance which is serving as the basis of the work. these difficulties are greatly augmented during the latter part of the work; because after the beginning of the quantitative experiment, not only must the substance be kept in a pure state, but also it must be collected to the last trace and brought on to the balance pan—a stern condition not imposed by the pre- liminary preparation of material. if any escapes collection, the loss must be estimated by careful experiments, so that its exact amount may be known. all the weights used: must be carefully standardised, and correction must be made for the buoyancy of the air on the bodies weighed. for further statement of these and other precautions and for a brief description of apparatus suitable for avoiding many pitfalls, together with the details of an especially instructive complex case, the reader js referred to carnegie institution of washington, publication no. 125. a critical summary by f. w. clarke of all investigations up to 1920 is to be found in the third memoir of volume 16 of the memoirs of the national academy of sciences (washington). a typical experiment.—a simple case may best exemplify the method. in one of many experiments, 7-59712 grammes of ferric oxide (fe203) prepared with the greatest care, were found to yield on reduction (by means of hydrogen at a high tem- perature) 5-31364 grammes of metallic iron. the loss of weight (2-28348 grammes) represents the oxygen present in the oxide. hence, from the proportion (2-28348) : (5-31364)=os3 : fe.= 3(16-000) : 2x, the atomic weight of iron is found to be 55-848. (g. p. baxter and c. r. hoover.) the analysis was repeated many times in order to eliminate accidental errors. silver is often used as an intermediary standard of reference, because its chloride, bromide and iodide are particularly suscep- tible to exact treatment. for example: in 11 concordant exper- iments an aggregate of 60-6479 grammes of exceedingly pure ferrous bromide (febre) were found to require 60-6731 grammes of the purest silver to precipitate all the bromine present. (bax- ter and thorvaldson.) the accepted atomic weight of silver being 107-88, we have the proportion: -—(60-6731) : (60-6479) = 2 ag: febre= (2[107-88]): y, giving y=215-670=febr. since br=79-916 (found by converting a known weight of silver into silver bromide), the atomic weight of iron is thus found to be 55-838, a value only slightly less than that found from the oxide. the rounded average of the two values (55: 84) 3 is taken as the true value. alternative method—another general method of determining atomic weights (applicable only to gases or vapours) depends upon avogadro’s rule, and resolves itself into the weighing of like volumes of different gases under like conditions of tempera- ture and pressure. this is the only gravimetric method appli- cable to the six inert gases (helium, etc.) which do not form chemical compounds. the method determines molecular weights, not atomic weights; but the number of atoms in a mole- cule may be inferred in other ways, and therefore the atomic weights calculated from the data. the method involves experi- mental difficulties. the globe containing a gas inevitably weighs much more than the gas itself and is peculiarly subject to changes of buoyancy of the air. the exact measurement of temperature and pressure is not always easy, nor is the perfect purity of the gas to be weighed a condition readily secured. moreover, avogadro’s rule holds only for perfect gases; no actual gas fulfils exactly its requirements, because of the bulk occupied by the molecules themselves and their mutual attrac- tion. on the whole, making allowance for these difficulties, the method of determining molecular (and therefore atomic weights) by comparison of the densities of gases agrees remarkably well with the results obtained from chemical analysis (lord ray- leigh, e. w. morley, p. a. guye, a. leduc, e. moles, g. p. baxter). third method.—a third method of determining atomic weights (like the last, a purely physical method) is that which deter- mines the mass (or rather the ratio of mass to electric charge) of 269 rapidly moving charged atoms or molecules by means of their deflection by electric and magnetic fields. it appraises with moderate precision (by means of impressions on a photographic plate of the positions of impact of the deflected particles) the relative atomic masses pertaining to selected groups of atoms. in its original form it furnished the first experimental evidence not only that in some elements the atoms are all alike in weight, but also that in other elements this is not the case (sir j. j. thomson, 1912). different varieties of a single chemical element, similar in every respect except as regards the weights and masses of their atoms, and apparently inseparable by natural agencies when once mixed, are called isotopes (1°. soddy). under that head will be found a full description of this method of evaluating them, which was greatly improved by f. w. aston, in his ‘ mass-spectrograph ”’ (see isotopes)... isotopes —many but not all of the elementary substances have been found by this third method to be isotopic or ‘‘ com- plex.” hence elements may be divided into two classes: simple elements, probably possessing only one variety of atom, and isotopic elements, containing two or more varieties. the relative proportions of the several isotopes in a given elementary sub- stance are shown roughly by the relative intensities of the “ photographic” records; they can be shown exactly only by quantitative analysis, and then only when no more than two isotopes are present. thus ordinary terrestrial chlorine (cl= 35:46) must consist of a mixture of about 30 atoms of (cl=37) ta every 100 atoms of (cl=3s), | although the term “atomic w eight” referred originally to the elementary substances (whether simple or isotopic) which actually occur on the earth’s surface, it is applicable with even greater fitness to each isotope alone. of all the isotopic elements only one, namely lead, has had the atomic weight of any in- dividual isotope accurately determined by chemical analysis (richards, soddy, henigschmid). the individual isotopes of this metal are unique, because, so far as we can tell, they are end- products of the spontaneous disintegration of uranium, and other radioactive elements, in which the atoms of lead were segregated at the moment of their terrestrial birth and confined in the minerals producing them. their abnormal atomic weights (determined by chemical methods of unquestioned trustworthi- ness) constitute the most convincing evidence of the existence of isotopes. for the determination of the atomic weights of the isotopes of all the other isotopic elements, some form of “ mass- spectrograph ” is needed because complete separation of mixed isotopes is at present unattainable in any other way. table of atomic weights—the following table of atomic weights of the chemical elementary substances as they exist on the surface of the earth is essentially the table issued in 1925 by the international committee on elements and atomic weights, but includes the newly discovered element hafnium, as well as two of the individual isotopes of lead which have been experimentally investigated by chemical methods. “atomic numbers” are also given. usually, the larger the atomic weight, the larger the atomic number; but all isotopes of a given element have the same atomic number. except in the case of hydro- gen, the atomic number is never more and usually somewhat less than half of the atomic weight. the following table records quantities of outstanding practical importance; it is the trade mecum of the analytical chemist. its value to theory also is very great. redefinition of term.—the discovery of the spontaneous dis- integration of radioactive elements and the finding of isotopes have modified our theoretical interpretation of the atomic weights. because of these discoveries, two a priori premises (of a more or less philosophical nature), namely, first, the assumption that the atoms are indivisible (the elementary substances being ab- solutely permanent) and, second, the assumption that the atoms of a given chemical element are all alike in weight, must to-day be abandoned, but the premises are seen on close scrutiny to be by no means an essential part of the chemical atomic theory. nevertheless, the old definition of atomic weights must be altered in order to correspond exactly to modern knowledge. atomic weights table of atomic weights of the chemical elements 270 symbol | at. no. at. wt. aluminium al 13 26-97 antimony sb 5i 21°77 argon e. a 18 39°91 arsenic . . . » as a4 74:96 barium . b 6 137°37 beryllium (glucinum) | be a 902 bismuth : bi 83 209 ‘00 boron , b 5 10:82 bromine ‘ br a5 79916 cadmium . . .-|. cd 48 112-41 calcium ; ce ca 20 40-07 carbon . ‘ : i hie ‘ss 6 ‘12-000 cerium . ge ce 58 140°25 cesium (caesium) a cs 55 132-81 chiorine et cl 17 35°457 chromium cr 24 52-01 cobalt. co 27 58-94 columbium (niobium) cb 41 93:1 copper . : f cu 29 63.57 dysprosium . dy 66 162+52 erbium . | er 68 167°7 europium eu 63 152-0 fluorine : - . ane ae ef 9 19-00 gadolinium .° . e gd 64 157-26 gallium . _ ‘ ga 37 69:72 | germanium . oe ge a2 72-60 gold ©. 2. eee au 79 197-2 hafnium (celtium) ae hf 72 180: helium . . eo a he 2 4:00 holmium ‘ ; , ho 67 163-4 iwydrogen 2 ee h i 1-008 indies 6 «7 « «© & in 49 114-8 jodine . : ; e : i 53 126932 iridium . ir 77 193°1 jron ae an fe 26 55°84 krypton x. 4 kr 36 82:9 lanthanum . : la 57 138-90 lead (ordinary) . : pb 82 207-20 * frome g): a f . i 82 206-06 ‘* (from th) . a a 82 208- lithium : : : li 4 6-940 luteclum . . . . lu 71 175-00 magnesium . mg 12 24°32 a more complete and precise definition may be worded as follows: “ primarily, atomic weights are appropriate simple multiples (decided by theory) of the relative combining propor- tions or relative gas-densities of elementary substances calcu- lated on a consistent basis. they represent the relative average weights of the atoms of given specimens of elementary sub- stances referred to a common standard.” any such definition involves other definitions. an elementary substance is a sub- stance which is not disintegrated into other elementary sub- stances by ordinary chemical reactions. this definition avoids the implication that such a substance is incapable of disintegra- tion by extra-chemical means. ‘‘ element ” and “ chemical ele- ment” are sometimes used synonymously. ‘‘ atoms” are postulated as the smallest particles of such a substance under ordinary conditions. they are not necessarily incapable of dis- integration under extreme conditions. hence their name (from & privative and ropds ‘ divided, cut ”) is not now appropriate, but it will doubtless be retained; the term ‘‘ chemical atom ” would perhaps be better. the qualification involved in the word “ average ” above is necessary because of the discovery of isotopes. the weighted average of the atomic weights of the isotopes in any particular isotopic or “ complex” elementary substance is that which is recorded as its atomic weight. constancy of atomic weights ——that the atomic weights are constant in different compounds is shown by the analysis of many pure substances containing the same element and also by | symbol | at. no. | at. wt. manganese . mn 25 54°93 mercury he 80 200-61 molybdenum mo 42 96-0 neodymium nd 60 44°27 neon ne 10 20°2 nickel. ni 28 58-69 nitrogen (azote) n 7 14-008 osmium os 76 190-8 oxygen o 8 16-000 palladium pd 46 106°7 phosphorus . p 15 31°027 platinum ; 2 , rt 7; 195-23 potassium (kalium) . . k 19 39 -096 praseodymium pr 59 140-92 radium. : ra 88 225-95 radon (niton) rn 86 222: rhodium rh 45 102-91 rubidium rb 37 85:44 ruthenium . ru 44 101-7 samarium sm 62 150-43 scandium sc 21 45:10 selenium se 34 79°2 silicon . si 14 28-06 silver. ag ag 107 880 sodium (natrium) na 11 22-997 strontium sr 38 87°63 sulphur s 16 22-064 tantalum ta 73 181-5 tellurium te 52 127°5 terbium tb 65 159°2 thallium ti si 204-39 thorium th 90 232-15 thulium tm 69 169-4 tin sn 50 118-70 titanium , ti 22 48-1 tungsten (w olfram) : w 74 184-0 uranium u 92 238-17 vanadium v 23 50-96 xenon , xe 54 130-2 ¥tterbium yb 70 173°6 yttrium a y 39 88-9 zine zn 30 65°38 zirconium zr 40 gi- h. landolt’s experiments (1907), which proved that there is no loss or gain of gravitational effect in ordinary chemical reactions within one part in ten million. moreover, specimens of various elementary substances (e.g., sodium, calcium, copper, silver, iron, nickel, cobalt, etc.) found in different parts of the earth or even in meteorites, have been found by careful research to have constant atomic weights independent of geographical occurrence. all the samples of terrestrial lead even, except those found in uranium or thorium minerals, show similar uniformity. that each native terrestrial mixture of isotopes is thus unvarying seems to show that each was commingled when the earth was still fluid, or else that some unknown law determines the proportion in which the isotopes are formed. if it were not for the consist- ency indicated in this paragraph, the table of atomic weights would be much less useful than it is. the atomic weights are precisely consistent also with the electro-chemical equivalents indicated by faraday’s law (faraday, rayleigh, richards), afiording thus further evidence of their fundamental nature. hydrogen and other elements.—the hypothesis of prout (1815) that all the elements are aggregates of hydrogen has been greatly strengthened by the discovery of isotopes; for it appears that the fractions in the table above are due chiefly to isotopic mixtures, in which each isotope taken separately has nearly a whole number for its individual atomic weight. the atomic weights of uranium, radium, thorium, the isotopes of lead, and helium furnish an argument in favour of the theory of the auckland—australia atomic disintegration in which they are concerned, and there- fore support the postulate maintaining the composite nature of the elements. nevertheless, all the simple elements and in- dividual isotopes have atomic weights somewhat less than the appropriate multiples of that of hydrogen, as has been shown in the case of oxygen. many theorists believe that this common deficiency is due to the actual loss of mass during the atomic coalescence of hydrogen nuclei, the expelled mass being trans- formed into energy. if this is true, the exact values of the simple atomic weights (and those of individual isotopes) even to the third decimal place, possess great theoretical interest, since they must furnish an essential clue to the amount of en- ergy expended. modern hypotheses concerning the structure of the atom (sir e. rutherford, sir j. j. thomson, n. bohr, g. n. lewis, i. langmuir) assume that practically all the weight and mass of the atom (fixing, of course, its atomic weight) are concentrated in an exceedingly small nucleus in its centre. concord with atomic numbers.—for fifty years the atomic weights decided the arrangement of the periodic system of the elements. recently x-ray spectra have more certainly evaluated the atomic numbers which place the elements in this system (moseley); but the agreement between the two methods is close enough to indicate a fundamental if sometimes complex relation between them. atomic weights and cosmogony.—the sun and stars appear spectroscopically to be made largely of the elements existing on earth. itis therefore no mere flight of fancy to infer that the vast gravitational forces which regulate the motions of the heav- enly bodies are due to the collective action of countless myriads of atoms, whose individual shares in the process are recorded in the table of atomic weights. the foregoing considerations con- cerning atomic weights suggest many other cosmological infer- ences, which are, however, beyond the scope of this article (see ** atomic weights and isotopes,” chemical review, i, 1, 1924). it is not too much to say that these unique numbers, the atomic weights, probably bear a very close relation to the unknown fundamental processes which determined the nature and evolu- tion of the universe. (t. w. r.) auckland, new zealand (see 2.894), increased largely in area, and the population rose from 82,101 in 1906 to 172,935 in 1924. from 1913 to 1926 seven outlying boroughs were in- cotporated in the city boundaries, making the area 7,844 acres. many streets have been paved and the traflic routes widened and improved. the grafton bridge was erected in 1910, a town hall in 1911 and a new building for the state maternity hospital in 1923. in 1913 the art gallery was enlarged, and an old colonists museum, with objects illustrating the history of new zealand and of auckland in particular, was installed therein in 1916. a number of parks and reserves have been acquired by the municipality, which now controls 8,000 ac. of open spaces. in the wakefield street reserve is a memorial of the early wars in new zealand, unveiled in 1920, and the foundation-stone of the world war memorial was laid in 1925 in auckland domain, which has been improved through the use of the profits from the industrial exhibition held there in 1913-4. a winter garden has been built and a statue of robert burns was unveiled in the park in 1921. a large dam was constructed at nikotupu in 1925 to augment the water supply. before the world war several wharves and a western breakwater were built in the harbour and land was reclaimed in freeman’s bay. since 1919 further work has been carried out, and parts of the harbour have been deep- ened. there are 14,925 ft. of wharfage. auction bridge: see bridge. auffenberg-komarow, moritz, ritter von (1852- ), austrian general, was born may 22 1852 at trop- pau. a most able soldier, auffenberg was one of the leaders of the austrian military party, which centred round the archduke franz ferdinand. owing to the latter’s influence he became min- ister of war in sept. rgor1 until dec. 1912, when he resigned owing to the opposition of the emperor and the magyars. in 1914 he commanded the austrian iv. army and won a remarkable victory at komarew, aug. 26—sept. 3 1914, but was suspended 27k for alleged irregularities in april rors. he published an autobiography, <tus oesterreichs hehe und niedergang (1921). austin, alfred (1835-10913), british poet (see 2.938), died june 2 1913 at swinford old manor, near ashford, kent. austin, mary hunter (1868- ), american author, was born in carlinville, ill., sept. 9 1868. after graduating from blackburn university in 1888, she went to california for reasons of health, marrying there stafford w. austin of bakersville, cal., may 19 1891. for some years she resided at independence, cal., and her first book, the land of little rain (1903), dealt with outdoor life in california, as did many of her succeeding works. she made a special study of the indians of the southwest, embodying the results in many articles and addresses. among her best known works are the flock (1906); lost borders (1909); the arrow maker, a play produced at the new theatre, n.y., 1911; the man jesus (1915; revised edition, 1925, entitled a small town man); and the american rhythm (1923). she also wrote the chapter on “‘ aboriginal literature ” in the cambridge history of american literature (1919).