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ly, those which are the best conductors of the electric fluid are also the best conductors of this; and è contra.
Thus a body which is a good conductor of fire readily receives it into its substance, and conducts it through the whole to all the parts, as metals and water do; and if two bodies, both good conductors, one heated, the other in its common state, are brought into contact with each other, the body which has most fire readily communicates of it to that which had least, and that which had least read. ily receives it, till an equilibrium is produced, Thus, if you take a dollar between your fingers with one hand, and a piece of wood of the same dimensions with the other, and bring both at the same time to the flame of a candle, you will find yourself obliged to drop the dollar before you drop the wood, because it conducts the heat of the can. dle sooner to your flesh. Thus, if a silver teapot had a handle of the same metal, it would conduct the heat from the water to the hand, and become too hot to be used; we therefore give to a metal teapot a handle of wood, which is not so good a conductor as metal. But a China or stone ieapot, being in some degree of the nature of glass, which is not a good conductor of heat, may have a handle of the same stuff. Thus, also, a damp, moist air shall make a maņ more sensible of cold, or chill him more than a dry air that is colder, because a moist air is fitter to receive and conduct away the heat of his body. This fluid, entering bodies in great quantity, first expands them, by separating their parts a little ; afterward, by farther separating their parts, it renders solids fluid, and at length dissipates their parts in air. Take this fluid from melte ed lead or from water, the parts cohere again; the first grows solid, the latter becomes ice: and this is sooner done by the means of good conductors, Thys, if you take, as I have done, a square bar of
lead, four inches long and one inch thick, together with three pieces of wood planed to the same dimensions, and lay them on a smooth board, fixed so as not to be easily separated or moved, and pour into the cavily they form as much melted lead as will fill it, you will see the melted lead chill and become firm on the side next the leaden bar some time be. fore it chills on the other three sides in contact with the wooden bars, though, before the lead was poured in, they might all be supposed to have the same degree of heat or coldness, as they had been exposed in the same room to the same air. You will likewise observe, that the leaden bar, as it has cooled the melted lead more than the wooden bars have done, so it is itself more heated by the melted lead. There is a certain quantity of this fluid, called fire, in every living human body; which fluid being in due proportion, keeps the parts of the flesh and blood at such a just distance from each other, as that the flesh and nerves are supple, and the blood fit for circulation. If part of this due proportion of fire be conducted away, by means of a contact with other bodies, as air, water, or metals, the parts of our skin and flesh that come into such contact first draw more near together than is agreeable, and give that sensation which we call cold; and if too much be conveyed away, the body stiffens, the blood ceases to flow, and death ensues. On the other hand, if too much of this fluid be communicated to the flesh, the parts are separated too far, and pain ensues, as when they are separated by a pin or lancet. The sensation that the separation by fire occasions we call heat or burning. My desk on which I now write, and the lock of my desk, are both exposed to the same temperature of the air, and have, therefore, the same degree of heat or cold: yet if I lay my hand successively on the wood and on the metal, the latter feels much the coldest; not that it is really so, but, being a better
conductor, it more readily than the wood takes away and draws into itself the fire that was in my skin. Accordingly, if I lay one hand part on the lock and part on the wood, and after it had laid on some time, I feel both parts with my other hand, I find the part that has been in contact with the lock very sensibly colder to the touch than the part that lay on the wood. How a living animal obtains its quantity of this fluid, called fire, is a curious question. I have shown that some bodies (as metals) have a power of attracting it stronger than others; and I have sometimes suspected that a living body had some power of attracting out of the air, or other bodies, the heat it wanted. Thus metals hammered, or repeatedly bent, grow hot in the bent or hammered part. But when I consider that air, in contact with the body, cools it ; that the surrounding air is rather heated by its contact with the body; that every breath of cooler air drawn in carries off part of the body's heat when it passes out again; that, therefore, there must be in the body a fund for producing it, or otherwise the animal would soon grow cold; I have been rather inclined to think that the fluid fire, as well as the fluid air, is attracted by plants in their growth, and becomes consolidated with the other materials of which they are formed, and makes a great part of their substance; that, when they come to be digested, and to suffer in the vessels a kind of fermentation, part of the fire, as well as part of the air, recovers its fluid, active state again, and diffuses itself in the body, digesting and separating it; that the fire, so reproduced by digestion and separation, continually leaving the body, its place is supplied by fresh quantities, arising from the continual separation; that whatever quickens the motion of the fluids in an animal quickens the separation, and reproduces more of the fire, as exercise; that all the fire emitted by wood and other combustibles, when burning, existed in them before in a solid state, being only discovered when separating; that some fossils, as sulphur, seacoal, &c., contain a great deal of solid fire; and that, in short, what escapes and is dissipated in the burning of bodies, besides water and earth, is generally the air and fire that before made parts of the solid. Thus I imagine that animal heat arises by or from a kind of fermentation in the juices of the body, in the same manner as heat arises in the liquors preparing for distillation, wherein there is a separation of the spirituous from the watery and earthy parts. And it is remarkable, that the liquor in a distiller's vat, when in its best and highest state of fermentation, as I have been informed, has the saine degree of heat with the human body: that is, about 94 or 96.
Thus, as by a constant supply of fuel in a chimney you keep a warm room, so by a constant supply of food in the stomach you keep a warm body ; only where little exercise is used the heat may possibly be conducted away too fast; in which case such materials are to be used for clothing and bedding, against the effects of an immediate contact of the air, as are in themselves bad conductors of heat, and, consequently, prevent its being communicated through their substance to the air. Hence what is called warmth in wool, and its preference on that account to linen, wool not being so good a conductor; and hence all the natural coverings of animals to keep them warm are such as retain and confine the natural heat in the body by being bad conductors, such as wool, hair, feathers, and the silk by which the silkworm, in its tender embryo state, is first clothed. Clothing, thus considered, does not make a man warm by giving warmth, but by preventing the too quick dissipation of the heat produced in his body, and so occasioning an accumulation.
There is another curious question I will just yenture to touch upon, viz., Whence arises the sudden
extraordinary degree of cold, perceptible on mixing some chymical liquors, and even on mixing salt and snow, where the composition appears colder than the coldest of the ingredients ? I have never seen the chymical mixtures made, but salt and snow I have often mixed myself, and am fully satisfied that the coinposition feels much colder to the touch, and lowers the mercury in the thermometer more than either ingredient would do separately. I suppose, with others, that cold is nothing more than the absence of heat or fire. Now if the quantity of fire before contained or diffused in the snow and salt was expelled in the uniting of the two matters, it must be driven away either through the air or the vessel containing them. If it is driven off through the air, it must warm the air, and a thermometer held over the mixture, without touching it, would discover the heat by the raising of the mercury, as it must and always does in warm air.
This, indeed, I have not tried, but I should guess it would rather be driven off through the vessel, especially if the vessel be metal, as being a better conductor than air ; and so one should find the basin warmer after such mixture. But, on the contrary, the vessel grows cold, and even water, in which the vessel is sometimes placed for the experiment, freezes into hard ice on the basin. ·Now I know not how to account for this, otherwise than by supposing that the composition is a better conductor of fire than the ingredients separately, and, like the lock compared with the wood, has a stronger power of attracting fire, and does accordingly attract it suddenly from the fingers, or a thermometer put into it, from the basin that contains it, and from the water in contact with the outside of the basin; so that the fingers have the sensation of extreme cold by being deprived of much of their natural fire; the thermometer sinks by having part of its fire drawn out of the mercury; the basin grows colder to the