Section II.—Nature of Explosive Agents.
Mechanical Mixtures.
—In the preceding section, it was shown that an explosion is simply the rapid oxidation of carbon and hydrogen. To form an explosive agent, the problem is, how to bring together in a convenient form the combustible, carbon or hydrogen, and the oxygen required to oxidize it. Carbon may be obtained pure, or nearly pure, in the solid form. As wood charcoal, for example, that substance may be readily procured in any needful abundance; but pure oxygen does not exist in that state, and it is hardly necessary to point out that only the solid form is available in the composition of an explosive agent. In nature, however, oxygen exists in the solid state in very great abundance in combination with other substances. Silica, for example, which is the chief rock constituent, is a compound of silicon and oxygen, and the common ores of iron are made up chiefly of that metal and oxygen. The elementary constituents of cellulose, or wood fibre, are carbon, hydrogen, and oxygen; and the body known as saltpetre, or nitrate of potash, is compounded of potassium, nitrogen, and oxygen. But though oxygen is thus found in combination with many different substances, it has not the same affinity for all. When it is combined with a substance for which its affinity is strong, as in the silica and the iron oxide, it cannot be separated from that substance without difficulty; but if the affinity be weak, dissociation may be more easily effected. The former combination is said to be “stable,” and the latter is, in contradistinction, described as “unstable.” It will be evident on reflection that only those compounds in which the oxygen exists in unstable combination can be made use of as a constituent part of an explosive agent, since it is necessary that, when required, the oxygen shall be readily given up. Moreover, it will also appear that when one of these unstable oxygen compounds and carbon are brought together the mixture will constitute an explosive agent, since the oxygen which is liberated by the dissociation of the unstable compound will be taken up by the carbon for which it has a stronger affinity. Saltpetre is one of those compounds, and a mixture of this body with charcoal constitutes gunpowder. The means employed to dissociate the elements of saltpetre is heat. It is obvious that other compounds of oxygen might be substituted for the saltpetre, but this body being easily procurable is always employed. The chlorate of potash, for example, is less stable than the nitrate, and therefore an explosive mixture containing the former substance will be more violent than another containing the latter. For the violence of an explosion is in a great measure determined by the readiness with which the oxygen is given up to the combustible. But the chlorate is much more costly than the nitrate. As, however, the force developed is greater, the extra cost would perhaps be compensated by the increased effect of the explosion. But the instability of the chlorate is such that friction or a moderately light blow will produce explosion in a mixture containing that substance, a circumstance that renders it unfit to be the oxidizer in an explosive agent in common use. The nitrate is therefore preferred on the ground of safety. Saltpetre, or nitrate of potash, consists, as already pointed out, of the metal potassium in combination with the substances nitrogen and oxygen. Of these, the last only is directly concerned in the explosion; but the two former, and especially the nitrogen, act indirectly to intensify its effects in a manner that will be explained hereafter.
The chemical formula for nitrate of potash is KNO3, which signifies that three atoms of oxygen exist in this body in combination with one atom of nitrogen and one atom of kalium or potassium. As the atomic weights of these substances are 16, 14, and 39 respectively, the weight of the molecule is 101, that is, in 101 lb. of nitrate of potash there are 39 lb. of potassium, 14 lb. of nitrogen, and (16 × 3) = 48 lb. of oxygen. Hence the proportion of oxygen in nitrate of potash is by weight 47·5 per cent. It will be seen from this proportion that to obtain 1 lb. of oxygen, 2·1 lb. of the nitrate must be decomposed.
The carbon of gunpowder is obtained from wood charcoal, the light woods, such as alder, being preferred for that purpose. The composition of the charcoal varies somewhat according to the degree to which the burning has been carried, the effect of the burning being to drive out the hydrogen and the oxygen. But, generally, the composition of gunpowder charcoal is about 80 per cent. carbon, 3·25 per cent. hydrogen, 15 per cent. oxygen, and 1·75 per cent. ash. Knowing the composition of the charcoal, it is easy to calculate the proportion of saltpetre required in the explosive mixture.
Thus far we have considered gunpowder as composed of charcoal and saltpetre only. But in this compound, combustion proceeds too slowly to give explosive effects. Were the chlorate of potash used instead of the nitrate, the binary compound would be sufficient. The slowness of combustion in the nitrate mixture is due to the comparatively stable character of that body. To accelerate the breaking up of the nitrate, a quantity of sulphur is mixed up with it in the compound. This substance possesses the property of burning at a low temperature. The proportion of sulphur added varies from 10 per cent. in powder used in fire-arms, to 20 per cent. in that employed for blasting purposes. The larger the proportion of sulphur, the more rapid, within certain limits, is the combustion. Thus ordinary gunpowder is a ternary compound, consisting of charcoal, saltpetre, and sulphur.
As the composition of charcoal varies, it is not practicable to determine with rigorous accuracy the proportion of saltpetre required in every case; a mean value is therefore assumed, the proportions adopted being about—
| Charcoal | 15 |
| Saltpetre | 75 |
| Sulphur | 10 |
| 100 |
With these proportions, the carbon should be burned to carbonic acid, and the sulphur should be all taken up by the potassium. Powder of this composition is used for fire-arms. For blasting purposes, as before remarked, the proportion of sulphur is increased at the expense of the saltpetre, in order to quicken combustion and to lessen the cost, to 20 per cent. as a maximum. With such proportions, some of the carbon is burned to carbonic oxide only, and some of the sulphur goes to form sulphurous acid, gases that are particularly noisome to the miner.
It is essential to the regular burning of the mixture that the ingredients be finely pulverized and intimately mixed. The manufacture of gunpowder consists of operations for bringing about these results. The several substances are broken up by mechanical means, and reduced to an impalpable powder. These are then mixed in a revolving drum, and afterwards kneaded into a paste by the addition of a small quantity of water. This paste is subjected to pressure, dried, broken up, and granulated; thus, the mixing being effected by mechanical means, the compound is called a mechanical mixture. It will be observed that in a mechanical mixture the several ingredients are merely in contact, and are not chemically united. They may therefore be separated if need be, or the proportions may be altered in any degree. Mechanical mixtures, provided the bodies in contact have no chemical action one upon another, are stable, that is, they are not liable, being made up of simple bodies, to decompose spontaneously.