Explosives. An explosion may be occasioned by the sudden removal of resistance to an expanding force, as in the case of steam boilers; but it is more frequently the result of a sudden generation of energy by chemical reactions. Most explosions of this kind are instances of rapid combustion; and an explosive compound, as distinguished from a merely inflammable one, may be defined as one which contains within itself the elements of combustion or other chemical change, liberating mechanical energy. Thus the fire damp of coal mines, when pure, is inflammable; but mixed with a certain proportion of atmospheric oxygen, it becomes explosive. The ingredients of an explosive compound remain inert unless the condition of chemical reaction is supplied. This is usually heat, produced by the direct contact of a heated body, or by pressure or percussion. In some instances, however, the introduction of a new substance, or the change of aggregate condition in one or more of the ingredients, may occasion explosion. The number of explosives known to chemists is considerable.

Chiefly those which are employed in the arts will be considered in this article.- Gunpowder. Of these, gunpowder is the most widely employed, partly because the longest known, but mainly because it is not liable to spontaneous change, or explosion from other causes than a very high temperature (that of a spark or flame, for example), and because the manufacture can be cheaply carried on to any required extent, and can be so varied as to control the qualities of the product according to the proposed use. Gunpowder presents to the eye a mass of grains, usually angular and of uniform size, dark color, and polished surface. The different varieties range from 0.5 to 4.5 mm. in diameter of grain. Its specific gravity is 1.8 to 2.0. It explodes when rapidly heated above 300° C. It is composed of charcoal, sulphur, and nitre, the two former being the combustible ingre-dients, and the latter, by the surrender of its oxygon, supporting their combustion. According to the theory formerly held, the nitre is reduced during the combustion of rifle powder to nitrogen and potassium, the latter forming with sulphur potassium sulphide, while all the oxygen combines with the carbon of the charcoal to form carbon dioxide (carbonic acid). The formula expressing this reaction would be 2KNO3 + S + 3C=3CO + K2S + 2N; and the proportions of ingredients in 100 parts would be: nitre. 74.84; sulphur, 11.84; charcoal, 13.32. From blasting powder, on the other hand, carbonic oxide as well as carbonic acid is formed, and the theoretical reaction is shown in the equation KNO3 + S + 2C=KS + N + CO2 + CO, requiring; the proportions: nitre, 64.4; sulphur, 20.4; carbon, 15.2. How nearly these formulas are adhered to will appear from the following tables of analyses:

I. Military Powder

II. Rifle Or Sporting Powder

III. Blasting Powder

These variations are due partly to the variable quality of the ingredients, particularly the charcoal, which always contains water and ash. The best coal (from light non-resinous wood, like poplar, black alder, or willow) rarely contains over 83 per cent. of carbon. The composition of powder has been also varied from the theoretical formulas to obtain a variety in its effects, and the researches of Bunsen, Shishk off, Karolyi, Craig, and Fe-dorow have shown that the simple reactions upon which the formulas were based do not take place; that the products of combustion, which vary somewhat with the pressure under which ignition takes place, comprise, among the gases, small quantities of cnrbonic oxide, hydrogen, sulphuretted hydrogen, and free oxygen, and. in the smoke and residue, chiefly the sulphate and carbonate, not the sulphide, of potassium. Bunsen found the gases from rifle powder to be but 31'4 per cent. of the weight. The pressure generated by the combustion of gunpowder has been variously estimated.

Gatzsehmann gives the following table, compiled from different authorities:

The usual estimate at present is for rifle powder 4,000, and for blasting powder 2,000 atmospheres. It is believed that in practice half these figures are realized. The latest researches upon the heat set free by the combustion of powder, those of Roux and Sarran (Comptes Rendus, July, 1873), give the following results:

The time within which this pressure is developed is an important element in the practical effect. The particles of the powder are successively ignited and combustion becomes general. The rate of ignition is more rapid, and that of combustion is slower, the larger the grain of the powder. The finest-grained powder, when pressed closely together, behaves like a single mass, burning with comparative slowness, and hence showing less explosive power. It is employed in rockets and fireworks. For rifled guns, a coarse grain is now preferred, since its quick ignition gives the force required to press the projectile into the grooves, while its prolonged combustion augments the pressure until the projectile leaves the gun with maximum velocity. Blasting powder, which is required to lift and split, rather than to throw, is usually coarse-grained, though modern practice is tending to the employment of "quicker" powders; a change due to the observed effectiveness of the nitro-glycerine compounds. The composition of ordinary blasting powder, as above shown, effects a slow combustion.-A blasting powder now used to a considerable extent in this country contains Chili saltpetre (nitrate of soda) instead of nitre. It is unsuitable for sporting or military purposes.

Another variation from the usual formula is Oliver's powder, made in Pennsylvania, in which peat is substituted for charcoal, with increased safety of manufacture and cheapness of product. The West Virginia mineral grahamite, a hydrocarbon, has also been experimented upon as a substitute for charcoal, with favorable results. Common powder soaked at the moment of using in nitro-glycerine has been used in Swedish quarries, with trebled effectiveness. Dynamite is safer and better. Pyronene is a cheap, inferior blasting powder, made of 52.5 parts nitrate of soda, 20 parts sulphur, and 27.5 parts spent tan. In Davey's powder a part of the charcoal is replaced by flour, starch, etc, for safety in preparation. Slow-burning powders used in Germany (Neumeyer's, Kup's, etc.) contain less sulphur and more coal than the ordinary kind. They are recommended for safety and small amount of smoke.-An intimate mixture of 3 parts nitre, 2 parts dry carbonate of potassa, and 1 part sulphur will when slow-ly heated (e.g., in an iron spoon) first melt, and soon after explode with deafening noise.