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.
As the effect of caloric, according to their view, depends on motion, the agencies by which this is effected are of the first importance. That it exists in all bodies in a state of rest, and in a greater or smaller quantity, and consequently in a relative proportion, is well known, and on this, the capacities of bodies for caloric is founded. The capacities of bodies for heat are changed by various means, and caloric is put in motion; and, according to its quantity, the bodies may be either cold or hot. When the surrounding bodies become heated, they receive this caloric thus set free, and, in this view, the absolute quantity of their heat is increased. This state of rest, to which caloric is subject, may be destroyed either by an increase or a diminution of the capacity of a body. If caloric be put in motion by causes of any kind, which influence the capacities of different bodies, a theory maintained by Davy, then as the capacity for heat is changed so is free heat produced. Diminish the capacity of a body, its excess will of course be given out, and distribute itself among the surrounding bodies, which become heated; but increase the capacity, and a different effect ensues. The body absorbs caloric, by which its capacity is increased, and cold is produced. Caloric, whether considered a substance, or an attribute, possesses, nevertheless, this property, that when it is given out, as in the mixture of sulphuric acid and water, which occupies a less space than both in a separate state, the sensation of heat follows; and when it is absorbed, as in the various freezing mixtures, or in a mixture of snow and common salt, the sensation of cold is excited. The causes, however, which set caloric in motion, or that produce heat, are such as combustion, condensation, friction, chemical mixture, and the like. It is remarkable, that these effects are invariably the same, and are affected by corresponding affinities. When a piece of iron is struck with a hammer, the percussion produces a condensation of the iron, its specific gravity is increased, and the iron finally becomes ignited. The condensation of air, in the condensing syringe, will set fire to tinder. The flint and steel produce a condensation; for the metal, although small, is sent off in scintillations in the state of ignition. That caloric is contained in bodies in the state of absolute rest, and is evolved by condensation, there is no doubt. Gunpowder, by percussion, in contact with pulverized glass, is inflamed; and it appears very probable, that it also contains caloric in a state of rest. The experiments of Lavoisier and Laplace, on the quantity of caloric actually absorbed in nitric acid, and in a latent state, (noticed in the article on gunpowder), are satisfactory. If caloric is not in that state in nitre, how are we to explain the sudden transmission or evolution of caloric in fired gunpowder, where no external agent in any manner can influence the formation, or disengagement of caloric? Friction or attrition produces heat; and the distributable excess of caloric, as it is called, although not satisfactorily accounted for, may arise from a condensation; which, however, is denied.
The Esquimaux Indians kindle a fire, very expeditiously, in the following manner: They prepare two pieces of dry wood, and making a small hole in each, fit into them a little cylindrical piece of wood, round which a thong is put. Then, by pulling the ends of this thong, they whirl the cylindrical piece about with such velocity, that the motion sets the wood on fire, when lighting a little dry moss, which serves for tinder, they make as large a fire as they please; but as the little timber they have is drift wood, this fails them in the winter, and they are then obliged to make use of their lamps for the supply of their family occasions. Ellis's Voyage for the Discovery of a North-West Passage.
Friction is, therefore, one means of producing distributable heat, which is also exemplified very frequently in the axis of a carriage wheel; of mill work; in the rubbing of wood, when turned on its axis in a lathe, by which turners ornament their work with black rings; rubbing a cord very swiftly backwards and forwards against a post or tree, or letting it run over a boat, &c. as in the whale fishery; the motion of two iron plates against each other, pressing them at the same time, &c. The great object in the construction of machines is to avoid, or lessen the degree of friction. See Hatchette, Vince, and Gregory. Count Rumford (Nicholson's Journal, 4th edit. ii, 106), and professor Pictet (Essai sur le Feu, chap. ix.) have made some valuable experiments on heat evolved by friction.
The sun is one great source of caloric. In whatever mode it produces it, whether by giving it out from its own substance, by the action of light on the air that surrounds the globe; by the concentration of calorific rays by means of the atmosphere, acting as a lens; or by putting caloric in the distributable state, always pre-existing in some other, as in a state of rest, are questions, which, in our present state of knowledge, we are unable to solve. We know the fact, and that the caloric is of the same nature as that obtained by combustion.[3]
Combustion is a process by which caloric is put in a distributable state. The opinion of Stahl and others, that all combustible bodies contained a certain principle called phlogiston, to which they owed their combustibility, and that combustion was nothing more than a separation of this principle, gave rise to the phlogistic or Stahlian theory, which was afterwards modified by Dr. Priestley. But his theory is equally untenable. Kirwan's opinion was no less vague, although he substituted hydrogen for phlogiston.
The Lavoiserian, or antiphlogistic theory overturned the Stahlian. According to this theory, a combustible in burning unites with oxygen, and heat and light are given out by the gas, and not from the combustible. According to a modified theory of the above, by Dr. Thomson, caloric is evolved by the gas, and light from the burning body. Without noticing the instances, in which this theory, as a general one, is insufficient to explain the cause of combustion, or of the production of heat and light, we will merely remark,