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.
Chemical Compounds.
—In a mechanical mixture, as we have seen, the elements which are to react one upon another are brought together in separate bodies. In gunpowder, for example, the carbon is contained in the charcoal, and the oxygen in the saltpetre. But in a chemical compound, these elements are brought together in the same body. In a mechanical mixture, we may put what proportion of oxygen we please. But elements combine chemically only in certain definite proportions, so that in the chemical compound we can introduce only a certain definite proportion of oxygen. The oxygen in saltpetre is in chemical combination with the potassium and the nitrogen, and, as we have already seen, these three substances hold certain definite proportions one to another. That is, to every atom of potassium, there are one atom of nitrogen and three atoms of oxygen. Or, which amounts to the same thing, in 1 lb. of saltpetre, there are 0·386 lb. of potassium, 0·139 lb. of nitrogen, and 0·475 lb. of oxygen. Moreover, these elements occupy definite relative positions in the molecule of saltpetre. But in the mechanical mixture, the molecules of which it is made up have no definite relative positions. Even if the three substances—charcoal, saltpetre, and sulphur—of which gunpowder is composed, could be so finely divided as to be reduced to their constituent molecules, the relative position of these would be determined by the mixing, and it would be impossible so to distribute them that each should find itself in immediate proximity to those with which it was to combine. But so far are we from being able to divide substances into their constituent molecules, that when we have reduced them to an impalpable powder, each particle of that powder contains a large number of molecules. Thus, in a mechanical mixture, we have groups of molecules of one substance mingled irregularly with groups of molecules of another substance, so that the atoms which are to combine are not in close proximity one to another, but, on the contrary, are, many of them, separated by wide intervals. In the chemical compound, however, the atoms are regularly distributed throughout the whole mass of the substance, and are, relatively to one another, in the most favourable position for combining. Viewed from this point, the chemical compound may be regarded as a perfect mixture, the mechanical mixture being a very imperfect one. This difference has an important influence on the effect of an explosion. All the atoms in a chemical compound enter at once into their proper combinations, and these combinations take place in an inconceivably short space of time, while, in a mechanical mixture, the combinations are less direct, and are much less rapidly effected. This is the reason why the former is more violent in its action than the latter. The one is crushing and shattering in its effects, the other rending and projecting. The compound gives a sudden blow; the mixture applies a gradually increasing pressure. It is this sudden action of the compound that allows it to be used effectively without tamping. The air, which rests upon the charge, and which offers an enormous resistance to motion at such inconceivably high velocities, serves as a sufficient tamping.
Gun-cotton may be taken as an example of a chemical compound. The woody or fibrous part of plants is called “cellulose.” Its chemical formula is C6H10O5, that is, the molecule of cellulose consists of six atoms of carbon in combination with ten atoms of hydrogen and five atoms of oxygen. If this substance be dipped into concentrated nitric acid, some of the hydrogen is displaced and peroxide of nitrogen is substituted for it. The product is nitro-cellulose, the formula of which is C6H7(NO2)3O5. If this formula be compared with the last, it will be seen that three atoms of hydrogen have been eliminated and their place taken by three molecules of the peroxide of nitrogen NO2; so that we now have a compound molecule, which is naturally unstable. The molecules of the peroxide of nitrogen are introduced into the molecule of cellulose for the purpose of supplying the oxygen needed for the combustion of the carbon and the hydrogen, just as the groups of molecules of saltpetre were introduced into the charcoal of the gunpowder for the combustion of the carbon and the hydrogen of that substance. Only, in the former case, the molecules of the peroxide are in chemical combination, not merely mixed by mechanical means as in the latter. The compound molecule of nitro-cellulose may be written C6H7N3O11, that is, in 297 lb. of the substance, there are (6 × 12) 72 lb. of carbon, (7 × 1) 7 lb. of hydrogen, (3 × 14) 42 lb. of nitrogen, and (11 × 16) 176 lb. of oxygen; or 24·2 per cent. carbon, 2·3 per cent. hydrogen, 14·1 per cent. nitrogen, and 59·4 per cent. oxygen. When the molecule is broken up by the action of heat, the oxygen combines with the carbon and the hydrogen, and sets the nitrogen free. But it will be observed that the quantity of oxygen present is insufficient to completely oxidize the carbon and the hydrogen. This defect, though it does not much affect the volume of gas generated, renders the heat developed, as shown in a former section, considerably less than it would be were the combustion complete, and gives rise to the noxious gas carbonic oxide.
Cotton is one of the purest forms of cellulose, and, as it may be obtained at a cheap rate, it has been adopted for the manufacture of explosives. This variety of nitro-cellulose is known as “gun-cotton.” The raw cotton made use of is waste from the cotton mills, which waste, after being used for cleaning the machinery, is swept from the floors and sent to the bleachers to be cleaned. This is done by boiling in strong alkali and lime. After being picked over by hand to remove all foreign substances, it is torn to pieces in a “teasing” machine, cut up into short lengths, and dried in an atmosphere of 190° F. It is then dipped into a mixture of one part of strong nitric acid and three parts of strong sulphuric acid. The use of the sulphuric acid is, first, to abstract water from the nitric acid, and so to make it stronger; and, second, to take up the water which is formed during the reaction. After the dipping, it is placed in earthenware pots to digest for twenty-four hours, in order to ensure the conversion of the whole of the cotton into gun-cotton. To remove the acid, the gun-cotton is passed through a centrifugal machine, and subsequently washed and boiled. It is then pulped, and again washed with water containing ammonia to neutralize any remaining trace of acid. When rendered perfectly pure, it is compressed into discs and slabs of convenient dimensions for use.
Another important chemical compound is nitro-glycerine. Glycerine is a well-known, sweet, viscous liquid that is separated from oils and fats in the processes of candle-making. Its chemical formula is: C3H8O3; that is, the molecule is composed of three atoms of carbon, in combination with eight atoms of hydrogen, and three atoms of oxygen. In other words, glycerine consists of carbon 39·1 per cent., hydrogen 8·7 per cent., and oxygen 52·2 per cent. When this substance is treated, like cellulose, with strong nitric acid, a portion of the hydrogen is displaced, and peroxide of nitrogen is substituted for it; thus the product is: C3H5(NO2)3O3, similar, it will be observed, to nitro-cellulose. This product is known as nitro-glycerine. The formula may be written C3H5N3O9. Hence, in 227 lb. of nitro-glycerine, there are (3 × 12) 36 lb. of carbon; (5 × 1) 5 lb. of hydrogen; (3 × 14) 42 lb. of nitrogen; and (9 × 16) 144 lb. of oxygen; or 15·8 per cent. is carbon, 2·2 per cent. hydrogen, 18·5 per cent. nitrogen, and 63·5 per cent. oxygen. When the molecule is broken up by the action of heat, the oxygen combines with the carbon and the hydrogen, and sets the nitrogen free. And it will be seen that the quantity of oxygen present is more than sufficient to completely oxidize the carbon and the hydrogen. In this, the nitro-glycerine is superior to the nitro-cotton. In both of these compounds, the products of combustion are wholly gaseous, that is, they give off no smoke, and leave no solid residue.
In the manufacture of nitro-glycerine, the acids, consisting of one part of strong nitric acid and two parts of strong sulphuric acid, are mixed together in an earthenware vessel. When quite cold, the glycerine is run slowly into this mixture, which, during the process, is kept in a state of agitation, as heat is developed in the process; and, as the temperature must not rise above 48° F., the vessels are surrounded with iced water, which is kept in circulation. When a sufficient quantity of glycerine has been run into the mixture, the latter is poured into a tub of water. The nitro-glycerine being much heavier than the dilute acid mixture, sinks to the bottom; the acid liquid is then poured off, and more water added, this process being repeated until the nitro-glycerine is quite free from acid.
Nitro-glycerine is, at ordinary temperatures, a clear, nearly colourless, oily liquid, having a specific gravity of about 1·6. It has a sweet, pungent taste, and if placed upon the tongue, or even if allowed to touch the skin in any part, it causes a violent headache. Below 40° F. it solidifies in crystals.
Dynamite is nitro-glycerine absorbed in a silicious earth called kieselguhr. Usually it consists of about 75 per cent. nitro-glycerine and 25 per cent. kieselguhr. The use of the absorbent is to remove the difficulties and dangers attending the handling of a liquid. Dynamite is a pasty substance of the consistence of putty, and is, for that reason, very safe to handle. It is made up into cartridges, and supplied for use always in that form.