—In the combustion of gunpowder, the elements of which it is composed, which elements, as we have seen, are carbon, hydrogen, nitrogen, oxygen, potassium, and sulphur, combine to form, as gaseous products, carbonic acid, carbonic oxide, nitrogen, sulphuretted hydrogen, and marsh gas or carburetted hydrogen, and, as solid products, sulphate, hyposulphite, sulphide, and carbonate of potassium. Theoretically, some of these compounds should not be produced; but experiment has shown that they are. It has also been ascertained that the greater the pressure, the higher is the proportion of carbonic acid produced, so that the more work the powder has to do, the more perfect will be the combustion, and, consequently, the greater will be the force developed. This fact shows that overcharging is not only very wasteful of the explosive, but that the atmosphere is more noxiously fouled thereby. The same remark applies even more strongly to gun-cotton and the nitro-glycerine compounds.
The careful experiments of Messrs. Noble and Abel have shown that the explosion of gunpowder produces about 57 per cent. by weight of solid matters, and 43 per cent. of permanent gases. The solid matters are, at the moment of explosion, in a fluid state. When in this state, they occupy 0·6 of the space originally filled by the gunpowder, consequently the gases occupy only 0·4 of that space. These gases would, at atmospheric pressure and 32° F. temperature, occupy a space 280 times that filled by the powder. Hence, as they are compressed into 0·4 of that space, they would give a pressure of 2800·4 × 15 = 10,500 lb., or about 4·68 tons to the square inch. But a great quantity of heat is liberated in the reaction, and, as it was shown in a former section, this heat will enormously increase the tension of the gases. The experiments of Noble and Abel showed that the temperature of the gases at the instant of explosion is about 4000° F. Thus the temperature of 32° + 461°·2 = 493°·2 absolute, has been raised 4000493°·2 = 8·11 times, so that the total pressure of the gases will be 4·68 × 8·11 = 42·6 tons to the square inch. And this pressure was, in the experiments referred to, indicated by the crusher-gauge. When, therefore, gunpowder is exploded in a space which it completely fills, the force developed may be estimated as giving a pressure of about 42 tons to the square inch.
Relative Force developed by Gunpowder, Gun-cotton, and Nitro-glycerine.
—Unfortunately no complete experiments have hitherto been made to determine the absolute force developed by gun-cotton and nitro-glycerine. We are, therefore, unable to estimate the pressure produced by the explosion of those substances, or to make an accurate evaluation of their strength relatively to that of gunpowder. It should, however, be borne in mind that a correct estimate of the pressure produced to the square inch would not enable us to make a full comparison of the effects they were capable of causing. For though, by ascertaining that one explosive gives twice the pressure of another, we learn that one will produce twice the effect of another; yet it by no means follows from that fact that the stronger will produce no more than twice the effect of the weaker. The rending effect of an explosive depends, in a great measure, on the rapidity with which combustion takes place. The force suddenly developed by the decomposition of the chemical compounds acts like a blow, and it is a well-known fact that the same force, when applied in this way, will produce a greater effect than when it is applied as a gradually increasing pressure. But some calculations have been made, and some experiments carried out, which enable us to form an approximate estimate of the relative strength of these explosive substances.
Messrs. Roux and Sarrau give the following as the result of their investigations, derived from a consideration of the weight of the gases generated and of the heat liberated. The substances are simply exploded, and the strength of gunpowder is taken as unity.
| Substance. | Relative Weight of Gases. | Heat in Units liberated from 1 lb. | Relative Strength. |
|---|---|---|---|
| Gunpowder | 0·414 | 1316 | 1·00 |
| Gun-cotton | 0·850 | 1902 | 3·00 |
| Nitro-glycerine | 0·800 | 3097 | 4·80 |
The relative strength is that due to the volume of the gases and the heat, no account being taken of the increased effect due to the rapidity of the explosion.
Alfred Noble has essayed to appreciate the effects of these different explosives by means of a mortar loaded with a 32-lb. shot and set at an angle of 10°, the distances traversed by the shot being taken as the results to be compared. Considered, weight for weight, he estimates as follows the relative strengths of the substances compared, gunpowder being again taken as unity:—
| Gunpowder | 1·00 |
| Gun-cotton | 2·84 |
| Dynamite | 2·89 |
| Nitro-glycerine | 4·00 |
The relative strength, bulk for bulk, is, however, of greater importance in rock blasting. This is easily computed from the foregoing table and the specific gravity of the substances, which is 1·00 for gunpowder and compressed gun-cotton, 1·60 for nitro-glycerine, and 1·65 for dynamite. Compared in this way, bulk for bulk, these explosives range as follows:—