In this part we also comprehend the General Theory of Fire-works, which it may be proper to remark, we have drawn from the known effects of chemical action; so far, at least, as the laws, of affinity, which govern such action, are applicable to the subject. The importance of this inquiry, although having no relation to the mere manipulations of the artificer, can not be doubted; since a knowledge of chemistry has already improved the preparation of gunpowder, and its effects are now known to be owing to the formation of sundry elastic aeriform fluids. On this head, that of the application of chemistry to Pyrotechny, we claim so much originality, as, so far as we know, to have been the first, who applied the principles of chemistry.
It is not to be expected in every instance, that a rationale of the decomposition as it occurs, or the order in which it takes place, can be given with certainty; because, where a variety of substances enter into the same preparation, which is frequently the case, the affinities become complicated, and the laws of action for that reason indeterminate, and frequently anomalous. But, on the contrary, in a variety of primary operating causes, by which effects analogous in their nature result, decomposition of course being the same, the causes are well understood, and the effects are thereby known, and duly appreciated.
This, for instance, is the case with a mixture of nitrate of potassa, charcoal, and sulphur, in the proportion necessary to form gunpowder; for, it is known, that the explosive effects of powder are owing to the sudden production of a number of gases, which suffer dilatation by the immense quantity of caloric liberated at the moment of combustion. Although the production of caloric by the inflammation of gunpowder is a case, which cannot be explained by the present received theory of combustion, as we have noticed in that article; yet we know that it is a fact, and that caloric is generated by the decomposition of the powder.
If we consider the primary cause of this decomposition, we naturally inquire into the products of the combustion, and endeavour to account for the production of the elastic aeriform fluids. We know that carbon has the property of decomposing nitric acid, and also nitrate of potassa; for, when it is brought in contact in the state of ignition with nitre, a deflagration will ensue, and carbonic acid be formed. The quantity of this acid is in the direct ratio to the quantity of oxygen required to saturate a given quantity of carbon; and therefore, by employing certain proportions of nitre and charcoal, the latter will decompose the former, and by abstracting its oxygen, on the same principle form carbonic acid, while the azote, the other constituent of nitric acid, will be set at liberty. Nor is this all, if we consider the action of sulphur. The sulphur must unite with one portion of the oxygen to produce sulphurous acid gas, and also with the potassa of the nitre, and form a sulphuret, a compound necessary to be formed, before we can explain the production of sulphuretted hydrogen gas, which results from the decomposition of water contained in the nitre. There also results, as a product, sulphate of potassa. In considering these products at large, it would be necessary to go into detail; and, as we have descanted largely on its combustion in gunpowder, we accordingly refer the reader to the article on Gunpowder. It will be sufficient, however, to remark, that the agent, and consequently the cause, which produces the decomposition of nitrate of potassa, is carbon or charcoal. This, by uniting with the greater part of the oxygen of the nitre, produces, in a determinate proportion, carbonic acid gas. This gas, therefore, in conjunction with other gases, formed at the same time, all of which being expanded, causes what is denominated the explosive effect of gunpowder.
We have then a primary cause of the decomposition, and most of the effective force of gunpowder is owing to the carbonic acid; and it is found, that gunpowder made without sulphur is equally powerful as that with, since it adds nothing to its power.
Causes, therefore, chemically speaking, operate alike under similar circumstances. The materials made use of being equally pure, and used in the same proportion, the effect must necessarily be the same.
It is not only in the instance we have mentioned, but in every other, in which chemical action ensues, that this doctrine is tenable. We might, indeed, notice a number of cases of a similar kind; as, for instance, in the combustion of many incendiary preparations, as fire-stone, fire-rain, composition for carcasses, light-balls, and a variety of fire-works of the same kind. If we mix pitch, tar, tallow, &c. with nitrate of potassa, and burn the mixture, we have the combined action of two elementary substances, which enter into the composition of these bodies, namely, carbon and hydrogen. The products would be carbonic acid gas, and water; because the oxygen of the nitre would unite with the hydrogen, as well as the carbon. If we employ sulphur at the same time, another product would be sulphurous acid gas, and probably sulphuric acid; and if gunpowder be used, as in the fire-stone composition, then, besides these products, we would have those of the gunpowder.
As this subject, however interesting to the theoretical pyrotechnist, cannot be understood without a knowledge of chemistry, it is obvious, that that science is a powerful aid to pyrotechny. It is unnecessary to dwell on this head. We may add, nevertheless that, in order to understand the effect of all mixtures, or compositions made use of, it is necessary to consider the nature of the substances employed, and the manner in which chemical action takes place, and consequently the products, which determine in fact the characteristic property of each species of fire-work, and the phenomena on which it is predicated. All products of combustion depend on the substances thus decomposed, and by knowing the effects, we may readily refer them to their proper causes.
With respect to caloric, it may not be improper to offer some remarks.[1] The hypothetical element of phlogiston having given way to the antiphlogistic theory at present received, our ideas respecting caloric are predicated on facts. Caloric is a term, which expresses heat, or matter of heat. In pyrotechny, we have merely to consider it in a free, or uncombined state; but as the subject is interesting, we purpose to notice it very briefly under the following heads: viz. its nature; the manner it is set in motion; its tendency to a state of rest; the changes it produces on bodies; and the instruments for measuring its intensity.
As to the nature of caloric, different opinions are entertained. We know the effect of heat: if we touch a substance of a higher temperature than our bodies, we call it hot, and vice versa. The one is evidently the accession, and the other the abstraction, of caloric. The latter is merely relative as respects ourselves; for the effect depends on our feelings, and the sensation of hot or cold is therefore governed by them.[2] Caloric, however, is considered to be a substance, composed of inconceivably small particles; but count Rumford and sir H. Davy are of a contrary opinion, namely, that it depends upon a peculiar motion and not on a subtle fluid.