Now, as all these changes are capable of being accurately measured by a micrometer, we are furnished with the means, not only of ascertaining the relative conducting pow.
ers of all transparent bodies, but also of determining tho curve of density, whether the variation of refractive power is produced by heat, by pressure, or by the mutual penetra tion of two different fluids. The optical properties which were developed during these experiments, will be explain ed in our article OPTICS. Similar results may be obtained in a manner very different, but equally satisfactory. Let .AB, Fig. 10. No. 2. be a vessel having a plate of annealed glass, whose section is MN, placed horizontally near the surface 7/Z n of the fluid, and supported by a glass pillar P. When this vessel is properly exposed to polarised light, no appearance will be seen through the edges MN of the glass ; but if a heated iron is suspended, as in Fig. 8. the descent of the heat towards the surface MN, and its pro pagation through the glass plate, will he marked by beauti ful fringes of light parallel to MN. Hence we have an other method, and a very direct one, of measuring the con ducting powers either of transparent or opaque bodies, by the Chromatic thermometer. See GLASS, THERMOME TER ; and the Phil. Trans. 1816. p. 108.
We have but few remarks to make on the manner in which heat is transmitted through gases. From their phy sical constitution, especially from the facility with which their particles move among each other, we might infer that heat would be conveyed through them, more in the way that it is through fluids, by the action of currents, than from particle to particle, as in solids. We know also that air, like liquids, is expanded by heat, and that this expansion causes it to become specifically lighter, and thus forms ascending currents of warm, and descending currents of cold air. Rumford endeavoured to prove by experiment, that the passage of heat through air was entirely brought about by these currents, and that it was in itself a perfect non-conductor. (Essays, vol. ii. p. 410. et seq.) But his ex periments, and the reasoning founded upon them, although ingenious, do not decisively prove the point ; they only go so far as to show, that whatever impedes the motion of the air, retards its heating and cooling, and that the communi cation of heat to air is, in part at least, effected by the same means as it is in fluids. Berthollct has indeed advanced
some considerations in favour of an opinion the very oppo site to that of Rumford's, that air is a good conductor of heat, (Stat. Chim. tom. i. p. 467,) but they cannot he con sidered very decisive, and, in short, the question must be regarded as one which is still open to further discussion.
Before we conclude the subject of the properties of heat, we must make a few remarks upon the cooling of bodies. This process depends upon the combination of a variety of circumstances ; partly upon the tendency to radiation, part ly upon the conducting power of the substances in contact with the body to be cooled, and partly upon the presence of those currents, which we have described as existing in fluids and gases. Professor Leslie, in the course of his re searches, to which we have so often alluded, made some experiments on the cooling of water contained in vessels -that had differently radiating surfaces. He found that a globe of polished metal, when filled with boiling water, cooled to a certain degree in 156 minutes, while under the same circumstances, except that the globe was coated with lamp black, the water required only half that time for cool ing. In this experiment, however, the cooling depends partly upon radiation, and pth.tly upon the conducting pow er of the substance of which the vessel is formed ; and Pro fessor Leslie attempted to estimate the relative effect of these two operations. From previous trials, he knew what quantity of heat would have been abstracted by radiation alone ; and thus it was easy to calculate what portion of the heat was removed by radiation, and what by the conducting power of the vessel. The general result is, that the effect of radiation is more powerful than that arising from the conducting power ; for the lamp black, which is known to be a bad conductor, increases the cooling of the water by augmenting the radiation. As might be predicted, from what has been already related, when the globe was im mersed in water, the cooling went on at the same rate, whether the surface was clean, or was covered with the black paint. In this case the process depended entirely on the internal currents that were formed in the water, while the surface of the vessel has no effect upon it. Inquiry, p. 268. 316.