EXPLOSIVES

-a very large number of explosives is now known, and apart from warlike purposes they are used extensively for mining, quarrying, tunnelling, road making, and like purposes.. the nature of exploston—the phenomenon of explosion is accompanied usually by a loud report, and frequently by a destructive effect, due to the sudden jiberation of a large volume of gas. ixplosives are substances which are capable of resolving themselves, under suitable impulses, almost instantaneously into gases which occupy a very much larger volume than the original subsiances. the explosion is invariably accompanied by a con- siderable evolution of heat, which still further increases the expansion of the gases. in general, explosion is akin to combustion, inasmuch as it depends in most cases upon the combination of carbon and hydrogen with oxygen. when a combustible material such as charcoal burns in the air, the combination of carbon with oxygen necessarily takes place slowly, since the spent air must be con- tinuously swept away and replenished by fresh air. in a draught of air, the burning is accelerated. if we were to mix the charcoal with hquid oxygen there would be no such delay, and it would be possible for the combustion to take place almost instanta- neously. this is the essential nature of explosion, the sudden release of energy accompanied by a large volume of gas. in gunpowder we have a mixture of charcoal and sulphur, both of which are combustible, with saltpetre (potassium nitrate), which contains an excess of oxygen, and thus on ignition the oxidation of the carbon and sulphur can take place very rapidly. modern explosives-—the main advance in modern explosives lies in the discovery of substances in which each molecule con- tains, in addition to carbon and hydrogen, the oxygen necessary for its combustion. gunpowder is merely a mechanical mixture and, although the ingredients are very finely powdered and closely mixed, they still form separate particles. the ad- vantages of combining the oxygen with the combustible elements in one and the same substance are:— 7 1093 1. gunpowder contains a large proportion of useless potash, which is merely a vehicle for the oxygen. the self-contained ex- plosives on the other hand do not contain any such waste mineral matter, and hence have a greater store of useful energy. 2. gunpowder gives a cloud of smoke when fired from a gun in consequence of the potash which it contains. the use of explosives containing no mineral matter gives rise to “smokcless powders,” which permit of much more rapid firing. 3. the closer contact of the oxygen with the combustible ele- ments renders possible a much more rapid internal combustion, lealing to a greater violence of explosion in the so-called “high explosives.” the compounds which are mainly used as explosives are those containing “‘ nitro groups,” in which the oxygen is attached, somewhat loosely, to nitrogen. for instance trinitrotoluene (tnt) is represented by the chemical formula c;iis; (nos)s. the oxygen may be considered to be in a state of strain, as it is striving to pass from the nitrogen to the carbon and hydrogen, to which it has a much greater affinity. the question naturally arises: if the molecules of an explosive are in a state of internal strain, why do they not fall to pieces spontaneously? the explanation is that a certain amount of energy must be imparted to the molecules to detach the oxygen from the nitrogen, before it can unite with the carbon and hy- drogen. when the reaction has once started, the much greater energy given out in the oxidation of the carbon and hydrogen loosens the oxygen of the nitro groups in the surrounding mole- cules, and so the explosion is propagated throughout the mass. the amount of energy which is necessary to loosen the atoms in the molecule differs greatly in different explosives, and this determines their chemical and mechanical stability, and the ease with which they can be brought to explode violently. thus trini- trotoluene is very stable, whilst mercury fulminate is very sensitive, and requires only a slight shock to cause it to explode with great violence. if a small quantity of mercury fulminate be exploded in contact with a larger quantity of trinitrotolu- ene, the fulminate ensures that the explosion is effectively started, and thus overcomes the initial inertia of the trinitro- toluene. the explosion then propagates itself through the mass of the explosive with a very high velocity. this very rapid explosion is termed detonation, and its velocity may vary from about 3,000 to 8,o00 metres (2 to 5 m.) per second. classificution.—it is sometimes a matter for surprise that so many different explosives should be in use, and it may appear at first sight that the one aim in the study of explosives should be to obtain an explosive of the greatest possible power. in prac- tice, however, the greatest explosive violence is not always desired. in firing a shot from a gun, for instance, a very violent explosive would shatter the gun. for this purpose a more grad- ual and sustained pressure is required. this leads to the first important classification into (2) propellants, which are used to drive the shot from a gun, and (%) high explosives, which are used for their shattering effect, e.g., in bombs and shells, and for rock blasting, etc. among high explosives again, different degrees of violence are required. thus in blasting soft materials, a relatively slow, heav- ing effect is required, whilst a shattering effect is required for hard rock. in some cases, the degrees of sensitiveness and stabil- ity on storage are of great importance, in others less important. special conditions have to be met in certain cases; thus the danger of ignition of firedamp in coal mines has led to the creation of a distinct class of explosives which can safely be used in such mines. economic considerations are naturally of im- portance, and in many cases cheapness has to be balanced against efficiency. detonants such as mercury fulminate form a separate class of explosives. these are limited to a very few representa- tives, and are used in caps and detonators in conjunction with other explosives. manufacture.—the most important explosive compounds are made by the action of nitric acid on various organic compounds. two main classes of chemical compounds come into consideration as explosives, namely nitric esters, that is nitrates of carbon com- pounds, and true nitro compounds. the chemical reactions, which are utilised in the preparation of these two classes of 1094 compounds, may be typified by the formation of glycerol trini- trate or nitroglycerine, a nitric ester, and trinitrotoluene, a true nitro-compound. c3h;(oh)3+3hno; = c.h;(ono,); +3110 glycerol . glycerol trinitrate toluene trinitrotoluene the process of conversion to a nitro compound or nitric ester is commonly called “ nitration.” in practice sulphuric acid is mixed with the nitric acid, to take up the water formed in the reaction. glycerol trinitrate or “ nitroglycerine ” is made by the nitra- tion of glycerol (glycerine) and forms a heavy ycllow oil. glyc- erol dinitrate is also used to some extent. cellulose nitrates or ‘“ nitrocelluloses ”’ are made by the nitration of cellulose, gen- erally in the form of cotton waste. the cotton retains its fibrous condition during nitration, but can be reduced to a gelatinous condition by solvents such as acetone, or a mixture of ether and alcohol. advantage is taken of this in the preparation of smoke- less powders. the viscous jelly is forced through an orifice, so as to form long sticks or cords, and the solvent is then dried off. cordite contains both nitrocellulose and nitroglycerine, together with a little mineral jelly. other nitric esters can be made by the nitration of starch and sugar, but have only a limited application. the true nitro-compounds are all prepared by the nitration of coal-tar products of the benzene series. they are generally yellow solid compounds, and are used either as such, or mixed with nitrates, etc. the nitro-compounds chiefly used for explosive purposes are dinitrobenzene, di- and trinitrotoluene (tn), trini- trophenol (picric acid or lyddite) and nitronaphthalenes. for war purposes, trinitroxylene, trinitroanisol, hexanitro-dipheny]- amine and others have also been used. mercury fulminate, hg(cno)., is not a nitro-compound, but is nevertheless made by the action of nitric acid on mercury and alcohol. in all cases, the explosives require careful purification, all traces of acid being carefully removed. mixed explosives.—most industrial explosives consist of mix- tures of several constituents. as in gunpowder, the general principle is to mix substances containing an excess of oxygen with combustible materials. the choice of oxidising agents is very limited, the main ones being nitrates, chlorates and perchlorates. in many cases the mixtures are reinforced by admixture with explosives such as nitroglycerine, nitrocellulose or trinitro- toluene. a special type of mixed explosive, which has been increasingly used, is made by adding liquefied oxygen to car- bonaceous materials. the oxygen evaporates very quickly, and the mixture must thercfore be made on the spot, and exploded at once. properties and testing—apart from ordinary chemical and analytical tests, the following characteristics are examined :— 1. explosive properties. the power is measured by detonating the explosive in a cavity in a lead block and measuring the increase in the size of the cavity. the heat of explosion and volume of the explosion gases are measured in a calorimetric bomb. the velocity of detonation can be determined by means of an electric chronograph. 2. sensitiveness to shock and friction, a falling weight machine serves to ascertain the sensitiveness to shock, and various friction devices are in use. 3. chemical stability. this determines the safety on storage. numerous tests are available, the general principle being to heat the explosive under carefully regulated conditions, and to ascertain the extent to which decomposition occurs, either by chemical reac- tions depending on the presence of oxides of nitrogen, by the evolu- tion of gas or by the alteration in weight. other-tests are carried out to ascertain the safety of explosives under special conditions. thus the length of flash and its dura- tion are determined photographically; the safety of explosives in presence of firedamp is examined in experimental gallerics simulating the conditions in mines. bibliography.—tanner, r. c., manufacture and uses of explo- sives (1921); department of scientific and industrial research, (a) technical records of explosives supply, 1915-1018; (b) manufac- ture of tri-nitrotoluene (1921); (c) manufacture of sulphurte acid by explosives, military contact process (1922); (d) synthetic phenol and picric actd (1921); (e) manufacture of nitric acid from nitric and sulphuric acid 1922). o. w. willcox ‘‘explosives,” in roger’s manual of indus- trial chemistry (1924). (r. c.f.)