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 same 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 venture 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 composition 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.