When heat, in its uncombined state, radiates through air, or through a vacuum, it moves with a velocity which has not been accurately measured, but which, there is reason to suppose, is immensely great ; so that, with re. spect to any distances near the earth, it may be said to be infinite. Heat also passes through the most dense and solid bodies, as, for example, through metals ; but in this case its progress is prodigiously retarded. It is also found to move through these bodies with various degrees of ve locity, which do not seem to bear an enact ratio to any other of their properties, and which can only be ascertain ed by direct experiment. This, in opposition to the rapid transmission of heat through the air, or through a vacuum, has been styled its slow communication ; and the power which solids possess, of enabling heat to pass along then), has been called their conducting power. When solids are in contact, and are possessed of different degrees of heat, they immediately tend to produce an equilibrium of tem perature; which is not brought about, as in the former case, by radiation, but by the one directly abstracting a portion of heat from the other, conducting this portion through its own substance, and difThsing it equally among its particles. This faculty exists in bodies in very different degrees; but it is found that each individual body always preserves the same degree of this power, unless some change takes place in its chemical or physical condition, when a change is at the same time produced in its eon• ducting power. A familiar but a correct of the different degrees of this conducting power in bodies, may be noticed in the different effects that are produced upon me tal and upon glass. If a rod of each of these substances have one of its ends plunged into hot water, the metallic rod will soon become so thoroughly heated, through all its extent, that it will be impossible to apply the hand to the other extremity, while the glass rod will remain a long time in the water, before the upper end is sensibly affected. Hence we say, that metals are good conductors of heat, and that glass is a worse conductor of heat than metals. As a general principle, it may be stated, that the densest bo dies are, the best conductors ; but to this rule there are many exceptions. Upon the whole, however, the prin ciple seems to hold good in so many instances, that we may infer the existence of a necessary connexion between the density and the conducting power of bodies, and that when the ratio is not correctly maintained. it should be attributed to, the interference of some other principle. Thus it is remarked, that the same body, without having experienced any chemical, or any other physical change, except a difference in its state of aggregation, has its con ducting power increased or diminished, in proportion to its density, or to the contiguity of its particles. Rumford fount] that a solid piece of iron is a better conductor than the filings of the same metal ; and that wood is a better conductor than saw-dust.
Many experiments have been performed on the con ducting power of solids, the object of which was to as certain the amount of this power, and to learn whether it bore an exact ratio to any other physical or .chemical pro perty. Metals, as was remarked above, are some of the best conductors of heat, but they differ considerably among themselves in this respect. Ingenhousz instituted a simple, but ingenious process, for discovering their relative power, which consisted in providing himself with rods of different metals, all of the same diameter, and hav ing a certain length covered with a coating of wax of the same thickness. The other ends of the roads were then plunged to the same depth in a heated fluid ; and, ac cording to the quantity of wax that was melted, their conducting power was estimated. The best conductors were found to be silver, gold, tin, and copper, while pla tina,kon, and 'lead, seemed to be the worst : (Jsurn. Phys. torn. xxxiv. p. 8.) These experiments prove that the conducting power of the metals is not precisely in the same ratio with their density. Richmann of Petersburg endeavoured to ascertain the same point by a different process. He procured h011ow balls of the several metals ; into these he inserted th'e bulb of a thermometer, immersed the balls in boiling water, and observed the effect upon the mercury : (Comment. Petrop. vol. iv. p. 241.) The re sults do not precisely agree with those of Ingenhousz ; but although more elaborate, we think them less direct ; because in Richmann's other causes besides the mere effect of the conducting power might act upon the ther mometer, an objection which does not seem to apply to Ingenhousz's.
A set of interesting experiments was performed by Rumford on the conducting power of various animal and vegetable substances. The plan which he pursued was to provide himself with a glass cylinder, which terminated in a globe of somewhat more than an inch in diameter. The bulb of a thermometer was suspended in the centre of this globe, and was surrounded by the substance to be examined ; and the whole apparatus was then plunged into boiling water, and when raised to a certain elevation of temperature, it was immersed in a mixture of ice and water, and the time noticed which was required to bring it down to this degree of heat. The heat from the water must obviously pass through the substance upon which the trial was made, before it could reach the thermometer; therefore its conducting power was estimated by the effect which it produced upon this instrument. He began by observing the length of time necessary for raising the thermometer enclosed in the cylinder, when it was sur rounded only with air, and what length of time was also necessary to cool it, by afterwards immersing it in a mix ture of ice and water. He then successively introduced into the globe, similar weights of wool, cotton, silk, linen, down, and fur, and compared their effects with air, taken as a standard ; the two last bodies were found to be the worst conductors, and linen the best, a fact which agrees with our experience of their effect when used as cloath ing; for we know that their only operation, as producing warmth to the body, must depend upon their retaining time heat which is generated in it, and preventing its escape. The Count concludes, that the relative conducting power of these substances was in the inverse ratio of the quan tity of air interposed between the particles or fibres of which they are composed. He found that their non-con ducting power was not in proportion to the quantity of solid matter which they contain, and therefore could not be from any mutual attraction between the solid matter and the air ; for the power bore no ratio to the actual quantity of the substance, but obviously depended upon: the mariner in which it was arranged.
He performed a direct experiment on silk, which esta blishes this point. Equal quantities of raw silk, the ravel lings of spun silk, and twisted silk thread, were respec tively placed in the apparatus ; and their conducting pow ers were found to be in the proportion of 9, 111, and 13, lie afterwards examined how far the air itself in the globe and cylinder might be conceived to be the sole agent, when it appeared, by calculating the space which the different substances occupied, and comparing the effect of the bulk of air which would be left, with the effect of the instrument when entirely filled with air, that the pro cess of cooling was retarded by the mixture of the body with the air ; and hence he deduced the conclusion, that the presence of the solid matter impedes the motion of the air, and prevents those currents from being formed in it, which we shall afterwards find to act so important a part in the heating and cooling of fluids : (Phil. Trans. 1792, and Essays, vol. ii. p. 428.) At the time when Rumford published his Essay, the effect of the radiation of surfaces was not thoroughly understood, and it is en tirely neglected in the view which lie takes of the subject. It must, however, have had a considerable shale in the production of the phenomena ; and we may imagine, that the more the particles or fibres of the solid matter are dispersed through the mass of air, the greater number of rays will be interrupted by them. It is also probable there is a real attraction between air and the different substan ces that were examined, which must have had its share in affecting the nature of the results ; but whether the at traction differs in different substances, or whether the at tractive power of the air is precisely equal upon all of them, does not appear to be exactly ascertained. Some experiments were made by Meyer of Erlangen, on the conducting power or the different kinds of wood. He pro cured. bulls of them, in which he formed a cavity to re ceive the bulb of a thermometer ; the balls were then ex posed to the same temperature, and the effects upon the mercury were noticed. The experiments, although per formed with apparent accuracy, are defective in not making any allowance of the action of radiation. Lime, fir, alder, and oak, were the worst, while ash, apple, and ebony, were found to be the best conductors of heat. ./Inn. Chim. t. xxx. p. 32.