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, that bodies which support combustion are called supporters, as oxygen gas, chlorine gas, &c. and those, that undergo this change, are named combustible bodies.
The products of combustion may be fixed or gaseous, and either alkalies, oxides, or acids; or, when chlorine is the supporter, chlorides, &c. A few examples will be sufficient. By the combustion of metals, iron for instance, we obtain a fixed product, and in the present case an oxide of iron; by the combustion of antimony and arsenic, the antimonic and arsenic acids; by the combustion of charcoal, we have carbonic acid gas, a gaseous product; by the combustion of potassium or sodium, we obtain a fixed alkali, depending however on the quantity of oxygen; by the combustion of sulphur, phosphorus, &c. acids; and when metals are burnt in chlorine gas, chlorides are produced.
It is evident from facts, that, whatever theory may be assumed, combustion occasions the production of free caloric, or changes the condition of caloric, from quiescent to distributable heat. The conclusions drawn by Mr. Davy and others, appear to have been predicated on the absorption of the base, and development of caloric, as in oxygen gas, and the peculiar alteration in bodies implying a decrease in their capacity; and hence, as regards the products of combustion, they must necessarily possess a less capacity for heat than the mean capacity of their constituents.
Whether we regard heat as latent, in the acceptation of the term, as applied or used by Dr. Black, or quiescent, or in a state of rest, it is certainly evident, that combustion is a chemical change, and by it caloric passes from a combined to an uncombined state, and is thus made sensible, free, or thermometrical heat. Combustion may, as it certainly does, put quiescent heat in a distributable state; but this quiescent heat is the same in the present case, of which there can be no doubt, as latent caloric. The thermometer will only indicate as much caloric in the air as is in a distributable, or free state; but, if the same air be employed to supply, or support combustion, the heat, rendered appreciable by the senses and the thermometer, will be in the ratio of the decomposition of the oxygen gas of the atmosphere, and, of course, to the development of free caloric.