Bodies then conduct caloric in conse quence of their affinity for it, and the pro perty which they have of combining in definitely with additional doses of it. Hence the reason of the slowness of the process, or, which is the same thing, of the long time necessary to heat or to cool a body. The process consists in an almost infinite number of repeated com positions and decompositions. We see, too, that when heat is applied to one ex tremity of a body, the temperature of the strata of that body must diminish equably, according to their distance from the source of heat. Every person must have observed that this is always the cage If, for instance, we pass our hand along an iron rod, one end of which is held in the fire, we shall perceive its tempera ture gradually diminishing from the end in the fire, which is hottest, to the other extremity, which is coldest. Hence the. measure of the heat-transmitted must al ways be proportional to the excess of temperature communicated to that side of the conductor which is nearest the . source of heat. The passage of caloric through a body by its conducting power must have a limit; and thatlimit depends upon the number of doses of caloric,with which the stratum of the body nearest the source of heat is capable of combin ing. if the length of a body be so great, that the strata of which it is composed exceed the number of doses of calorie With which a stratum is capable of com bining, it is clear that caloric cannot pos sibly be conducted through the body ; that is to say, the strata farthest distant from the source of heat cannot receive any increase of temperature. This limit depends, in all cases, upon the quantity of caloric with which a body is capable of combining before it changes its state. All bodies, as far as we know at present, are capable of combining indefinitely with caloric ; but the greater number, after the addition of a certain number of doses, change their state. Thus ice, after com bining with a certain quantity of caloric, is changed into water, which is convert. ed in its turn to steam, by the addition of more caloric. Metals, also, when heated to a certain degree, melt, are volatilized, and oxydated; wood and most other com bustibles catch fire, and are dissipated. As to the rate at which bodies conduct caloric, that depends upon the specific nature of each particular body, the best conductors conducting most rapidly, and to the greatest distance. When bodies are arranged into sets, we may lay it down as a general rule, that the densest set conduct at the greatest rate. Thus the metals condo t at a greater rate than any other bodies. But in considering the individuals of a set, it is not always the densest that conducts best : as bo dies conduct caloric in consequence of their affinity for it, and as all bodies have an affinity for caloric, it follows as a con sequence, that all bodies must be con ductors, unless their conducting power be counteracted by some other pro perty.
All solids are conductors ; because all solids are capable of combining with va rious doses of caloric before they change their state. This is the case in a very re markable degree with all earthy and stony bodies : it is the ease also with me tals, with vegetables, and with animal matters. This, however, must be under stood with certain limitations. All bo dies are indeed conductors; but they are not condnctors in all situations. Most so lids arc conductors at the common tem perature of the atmosphere ; but when heated to the temperature at which they change their state, they are no longer conductors. Thus, at the temperature
of 60°, sulphur is a conductor; hut when heated to 214°, or the point at which it melts or is volatilized, it is no longer a conductor. In the same manner ice con ducts caloric when at the temperature of 20°, or any other degree below the freez ;rig point; but ice at 32° is not a conduc tor, because the addition of caloric causes it to change its state.
With respect to liquids and gaseous bo dies, it would appear at first sight that they also are all conductors ; for they can be heated as well as solids, and heated too considerably without sensibly chang ing their state. But fluids differ from solids in one essential particular : their particles are at full liberty to onove among themelves, and they obey the smallest impulse ; while the particles of solids, from the very nature of these bo. dies, are fixed and stationary. One of the changes which caloric produces on bodies is expansion, or increase of bulk ; and this increase is attended with a pro portional diminution of specific gravity. Therefore, whenever caloric combines with a stratum of particles, the whole stratum becomes specifically lighter than the other particles. This produces no change of situation in solids; but in fluids, if the heated stratum happens to be be low the other strata, it is pressed up wards by them, and being at liberty to move, it changes its place, and is buoy ed up to the surface of the fluid. In fluids, then, it makes a very great dif ference to what part of the body the source of heat is applied. If it be applied to the Ugliest stratum of all, or to the surface of the liquid, the caloric can only make its way downwards, as through so lids,by the conducting power of the fluid; but if it be applied to the lowest stratum, it makes its way upwards, independently of that conducting power, in consequence of the fluidity of the body, and the ex pansion of the heated particles. The lowest stratum, as soon as it combines with a dose of caloric, becomes specifical ly lighter and ascends. New particles approach the source of heat, combine with caloric in their turn, and are displac ed. In this manner all the particles come, one after another, to the source of heat ; of course the whole of them are heated in a very short time, and the ca loric is carried almost at once to much greater distances in fluids than in any so lid whatever. Fluids, therefore, have the property of carrying or transporting ca loric ; in consequence of which they ac quire heat independently altogether of any conducting power.
If we take a bar of iron and a piece of stone of equal dimensions, and put ting one end of each into the fire, apply either thermometers or our hands to the other, we shall find the extremity of the iron sensibly hot long before that of the stone. Caloric, therefore, is not conduct ed through all bodies with the same ce lerity and ease. Those that allow it to pass with facility are called good con ductors; those through which it passes with difficulty are called bad conductors.
Metals are the best conductors of calo ric of all the solids hitherto tried. The conducting powers of all, however, are Rot equal. Dr. Ingenhousz procured cylinders of several metals exactly of the same size, and having coated them with wax, he plunged their ends into hot water, and judged of the conducting power of each by the length of wax-coating melt ed. From these experiments he conclud ed, that the conducting power of the metals which he examined were in the following order: Silver, Gold, nearly equal Tin, Platinum, Iron, much inferior to the others.