(From calor, heat). Caloric. Lavoisier, in giving his reasons for the adoption of this term, says, "All bodies are either solid, liquid, or in a state of aeriform vupour, according to the proportion which takes place between the attractive force inherent in their particles, and the repulsive powerof the heat acting upon them; or in proportion to the degree of heat to which they are exposed. It is difficult to comprehend the phenomena, without admitting them as the effects of a great and material substance, or very subtile fluid, which, insinuating itself between the particles of bodies, separates them from each other. This substance, whatever it is, being the cause of heat; or, in other words, the sensation, which we call warmth, being caused by the accumulation of this substance; we cannot, in strict language, distinguish it by the term heat, because the same name would very improperly express both cause and effect."he therefore gave it the names of igneous fluid, and matter of heat. These periphrastic expressions, however, lengthen physical language, render it more tedious, less distinct and correct, so that the cause of heat, or that fluid which produces it, has been distinguished by the term caloric, considered as the respective cause, whatever that may be, which separates the particles of matter from each other. See Elements of Chemistry, p. 5.

There is, however, an intermediate state of water in air, or rather approaching the form of air, which M. Lavoisier has not considered, viz. vesicular vapour. It contains a greater degree of specific heat than water, and less than either of the permanent elastic gases. Its form, however, does not seem wholly to depend on its heat, but on its electricity; by which it is repelled from the higher regions, and does not descend in rain. This is the state of water in fogs and in clouds; but as this subject admits of no application to medicine, we need not pursue it in this place.

We have anticipated the distinction of absolute and relative heat in our article on Calidum innatum, q. v. and shall now pursue its other effects.

When we speak of heat and its effects, we measure a very small part of an extensive scale. It is computed, though on no very secure foundation, that at about 1500° below the scale of Fahrenheit, it no longer exists; and we have in our power a degree equal to 32277° of that scale, the highest heat measured by Wedgewood's pyrometer. Our limits are between the 32d and the 120th degree of Fahrenheit, scarcely 88 degrees, yet even the effects of these changes arc interesting.

Expansion is one of the first and most striking effects. So far as it is applicable to the human body, we have noticed it under the article of Balneum, and may again advert to it under that of Heat. We there mentioned the blood as one of the least expansile fluids; but, as in the experiment some gas must escape, a little inaccuracy might be suspected. We had then in our view the experiments by Lavoisier, Prony, Guyton, and Prieur, on the expansility of different gases; of the considerable and equable expansility of carbonic acid gas; and the very great expansility of azotic gas in high temperatures. We find, however, from a Memoir of an ingenious chemist, Guy Lussac, an abstract of which occurs in the Annales de Chimie for 1802 (Thermidor, an X.), that when every cause of error is removed, particularly the presence of water, atmospheric air, oxygen, hydrogen, azote, nitrous, ammoniacal, carbonic, sulphureous, and muriatic acid gases, as well as the vapour of sulphuric aether, are dilated equally by the same degrees of heat; and that in the centigrade thermometer, from 0 to 80°, each dilated about 1/213 of its bulk for each degree. Of the fluids, the most expansile is nitric acid, then linseed oil, sulphuric acid, alcohol, water, and mercury, in their order. Of the metals, the expansility is nearly in the order of their fusibility, viz. zinc, lead, tin, pewter, brass, copper, bismuth, iron, steel, antimony, and platina. Of liquids the expansion is different, but few expand equably, viz. in equal degrees with equal increments of heats. Those which approach nearest to an equable expansion are mercury and alcohol, and are consequently preferred for filling thermometers. This effect of heat admits but of little-application in the practice of medicine. Cold applications in hernia and in topical inflammations, are the principal remedies which act in this way; though the latter admit of a somewhat different explanation.

Another effect of caloric, is the equilibrium which it affects: but this admits of modifications which we have already explained. The heat which raises one body a given degree, very slightly affects another; but to the touch and the thermometer the heat is in time the same. This law of heat chemists have found it difficult to explain. The popular idea, though not a correct one, may be the usual allusion of a sponge, which suffers the superabundant fluid to escape when its pores arc filled. This allusion also explains another effect, viz. when any body is dilated, heat is absorbed, when compressed, it escapes. Thus, in an exhausted receiver, if the air is humid, a cloud is formed on exhaustion. In a condensing engine we find heat escape sometimes rapidly; and, when suddenly dilated before the air can again absorb the free heat, even inflammation has taken place. We must repeat, however, that this allusion to the sponge is by no means correct. The equilibrium of heat depends rather on affinity, though apparently subject to some peculiar laws, and is little connected with physiology, as it relates to free caloric, and not to absolute or specific heats.

The laws of heat, most interesting to the chemical physiologist, relate to the powers of different substances in conducting heat. The motion of heat is slow, particularly when the conductors are fluids. Some authors, confounding heat with light, have given the former the velocity of the latter. They are, however, essentially distinct; and when air and water are interposed between small filaments of a solid, its motion is peculiarly slow. This renders feathers, eiderdown, and boiled mashed apples, bad conductors of heat: metals of every kind are, for the opposite reason, good conductors. We preserve the heat of the body by fur and eiderdown, and apply rasped potatoes to burns, which keep the part constantly cool. Count Rumford endeavoured to show-that water was a non-conductor of heat, and that it boiled in a vessel over the fire by successive currents coming in contact with the bottom. Such currents evidently exist, and explain the common paradox of the bottom of a kettle being cold while the water boils; but that water is a non-conductor of heat, can be by no means concluded from the experiment. On the contrary, Dr. Thomson has shown in Nicholson's Journal, vol. iv. p. 159, that water really conducts heat. Metals we have said are good, indeed they are the best, conductors. Of these, silver is better than gold, and this last metal excels copper and tin, which do not greatly differ. Pla-tina. iron, steel, and lead, are greatly inferior, and nearly in this order. Next follow stones, then glass, and afterwards dried woods, fine sand, charcoal (Annalesde Chimie, xxvi. 225,) feathers, silk, and wool, in the inverse ratio of their fineness. Of fluids, Dr. Thomson found an equal bulk of mercury to be twice as good a conductor of heat as water; and linseed oil somewhat better. It is highly probable, that the conducting power of bodies is in the ratio of their affinity for heat.