Caloric is the name given by modern chemists, to that substance by whose influence the phenomena of heat are produced, and which had before been distinguished by the terms igneous fluid, matter of heat, and other analogous denominations. In order to give precision to chemical language, it was necessary to adopt a word by which to distinguish between the substance which produces the sensation we call heat, and the sensation itself; these being connected as cause and effect; for whenever caloric becomes fixed in a body, it loses its property of affording heat. Whatever is the nature of that quality in bodies called heat, we are assured it does resemble the sensation of heat. A man whose mind is destitute of the cultivation of science, if endued with common sense, never imagines the sensation of heat to be in the fire; he only imagines that there is something in fire which occasions this sensation.
Though we are well acquainted with the effects of fire, we know but little of its nature. It is so active, as well as powerful a principle, that it eludes all our researches. We may, however, define it to be the phlogiston or inflammable principle, which pervades in a greater or less degree all substances. Boerhaave thinks it is a fluid of a nature peculiar to itself; that it was created such as it is, and cannot be altered in its nature or its properties; that it naturally exists in equal quantities in all places; and that it is wholly imperceptible to our senses, being only discoverable by such effects as in its operation it produces.[32]
That fire is really a substance, and not a quality, appears from its acting upon other substances, the reality of which has never been doubted. Charcoal, in its natural state, contains within its pores a large quantity of air; but if charcoal is heated, this air is expelled by the fire, which assumes its place, and occupies the pores of the charcoal. The burning of lime also, which deprives it of a great part of its weight by expelling the fixable air, demonstrates that fire, as a substance, enters into the pores of the lime, and forces out those other substances which are least intimately combined with it.
Collision or friction of solid bodies, is the means most generally used for exciting the action of fire. The vacuities of all solid bodies are replete with fire, so that it is impossible to agitate or separate their parts swiftly, without giving the same rapid motion to the element contained within them. When a piece of hardened steel is struck with a flint, some particles of the metal are scraped away from the mass, and so violent is the fire which follows the stroke, that it melts and vitrifies them. If the fragments of steel are catched upon paper, and viewed with a microscope, you will find most of them perfect spherules, and very highly polished. Their sphericity demonstrates that they have been in a fluid state, and the polish upon their surface shows them to be vitrified; the fire being disengaged with violence, disposes the particles of the substance to combine with the vital air, while this air accelerates the combustion. The whole of the heat produced is not afforded by the body itself, because in proportion as the interior fire is disengaged, the external air acts upon the body and gives out fire.
If the irons at the axis of a coach-wheel are applied to each other, without the interposition of some unctuous matter to keep them from immediate contact, they will become so hot when the carriage runs swiftly along, as to set the wood on fire; and the fore wheels being smallest, and making more revolutions, will be most in danger. The same will happen to mill-work, or any other machinery, if the necessary precautions are neglected. It is no uncommon practice with a blacksmith to use a plate of iron as an extemporaneous tinder-box; for it may be hammered on an anvil till it becomes red hot, and will fire a match of brimstone. A strong man who strikes quick, and keeps turning the iron, so that both sides may be equally exposed to the force of the hammer, will perform this in less time than would be expected. If in the coldest season you lay one dense iron plate upon another, and press the upper one, by a weight, on the lower one, and then rub the one over the other; by reciprocal motions, they will first grow warm, and at length so hot, as in a short time to emit sparks, and at last grow red hot, as if taken out of a vehement fire.
It is not necessary that the substance should be very hard; a cord rubbed backwards and forwards swiftly against a post or a tree will take fire; a stick of wood pressed against another which is turned swiftly about in a lathe, will soon make it turn black and emit smoke. Even the palms of your hands, if you rub them briskly together, when they are dry, will smell as if they were scorched. The method of exciting fire by rubbing two sticks of wood together, was anciently practised by country people, and is still retained in some parts of the world. The manner is exactly described in Captain Cook’s voyage. The inhabitants of New-Holland are there said to produce fire with great facility, and spread it in a wonderful manner. To produce it, they take two pieces of soft dry wood; one is a stick about eight or nine inches long, the other piece is flat. The stick they shape into an obtuse point at one end, and pressing it upon the other, turn it nimbly by holding it between both their hands, as we do a chocolate mill, often shifting their hands up and down, and then moving them down upon it to increase the pressure as much as possible. By this method they get fire in less than two minutes, and from the smallest spark they increase it with speed and dexterity.
The matter of fire is attracted more or less by all bodies. When any heated body comes in contact with a cold one, the former loses a part of its heat, and both of them become equally warm. If heated iron is laid upon a stone, its heat will flow into the stone; if thrown into the water, the heat will be diffused through the water. If a number of different substances, as metals, wood, wool, &c., are brought together into a place where there is not a fire, if they are of different temperatures, that is of different degrees of heat, the fire will be attracted from the hottest to those that are colder, till a perfect equilibrium is produced, or till they have all acquired the same temperature, as may be proved by applying the thermometer successively to each of them.
It does not appear, however, that all bodies have an equal attraction for the matter of fire. If a rod of iron is put into the fire for a short time, the end which is at a moderate distance from the fire will almost burn the hand; but a rod of wood, of the same length will be consumed to ashes at the end which is in the fire, before the other end is sufficiently heated to burn the hand. A ball of lead, and a ball of wool, may be of exactly the same temperature by the thermometer, but they will not appear of the same degree of heat on applying the hand. If they are of a temperature below that of our bodies, the lead will appear much colder than the wool, because it attracts the heat more rapidly from the hand; if they are of a higher temperature, the lead will appear much hotter, from the facility with which it parts with its heat. This property in bodies is called their conducting power; and those bodies through which the element of fire most rapidly circulates, are called good conductors.
The power of conducting the matter of fire seems to depend upon the texture of bodies, that is, upon the contact of their parts; hence the excessive slowness with which heat is communicated to bodies of a rare and spongy texture. Thus flannel, wool, and feathers, are considered as warm coverings, not because they possess more heat in themselves—for they serve to preserve any cold body in a cool state better than other substances—but because they prevent the escape of the animal heat from our bodies.
The matter of fire will exist in a state of combination, in a latent state, so as not to be perceptible to our senses. It will be found by observation, that every body which exists contains a quantity of the matter of fire in a fixed or neutralized state, disarmed of all its active, penetrating, and destructive qualities, like an acid and an alkali in combination.