means of conduction, and to be sure that the temperature was the same throughout. Thus it was still believed that the portion of heat transmitted through solid or liquid substances was governed by the same laws as the transmission of light, and that, cæteris paribus, the most diaphanous bodies transmitted the greatest quantity of caloric rays.
The results which I am about to state seem to me to establish beyond the possibility of doubt a fundamental proposition in the theory of radiant heat, namely, that the power of transmitting caloric rays is by no means proportioned to the transparency of the media; it is subject to a different law, which, in bodies without regular crystallization, appears to have many affinities to refrangibility. In crystals the phænomena are still more interesting, since in them we find that bodies possessing a high degree of transparency intercept nearly the whole of the caloric rays, while some others act in a manner directly contrary. These properties are invariably manifested whatever be the temperature of the source, and become yet more singular at low temperatures; for in the latter case we find that the ordinary heat of the hand passes immediately through a solid body of several inches in thickness. Let us not, however, anticipate as to the facts, but first of all examine the methods pursued in this third series of experiments.
In the first place it is unnecessary to dwell on the manner in which the solid screens have been exposed to the radiation and the indications of the thermomultiplier, for in this respect everything was the same as in the previous experiments. As to the liquids, these bodies are less permeable to radiant heat than solid bodies are. They must therefore be brought nearer to the thermoscope in order to obtain a well-marked transmission; but then the proper heat of the molecules themselves might be able to act on the instrument, and this the more certainly as the motions always developed in liquids unequally heated easily transfer the particles of the anterior to the further surface of the layers exposed to the source of heat. This effect of conductibility cannot be neutralized in a general manner by continually renewing the interposed layer, as in the experiments of M. Prevost; for some of the liquids can be procured only in small quantities; others, as soon as they are exposed to atmospheric air, undergo considerable alterations and evaporations which produce corresponding elevations or depressions of temperature that prove very annoying in experiments of this kind. The contrivance which I have employed for the purpose of avoiding these inconveniences is very simple. It consists in putting the liquids into very flat glass recipients, whose two large lateral surfaces are perfectly parallel, and the height four or five times that of the surface of the thermoelectrical pile. The lower part Of these vessels is applied to the mouth of the tube that envelops the face of the apparatus turned towards the source. The heat stopped by the anterior face of the vessel penetrates
