26 M. MELLONI ON THE FREE TRANSMISSION 



means of conduction, and to be sure that the temperature was the same 

 tliroughout. Thus it was still believed that the portion of heat trans- 

 mitted through solid or liquid substances was governed by the same 

 laws as the transmission of light, and that, cccteris 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 ra- 

 diant 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, ap- 

 pears to have many affinities to refrangibility. In crystals the phaeno- 

 inena are still more interesting, since in them we find that bodies pos- 

 sessing 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, how- 

 ever, anticipate as to the facts, but first of all examine the methods pur- 

 sued 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 Avell-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 atmo- 

 spheric 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 

 M'hich I have employed for the purpose of avoiding these inconve- 

 niences is very simple. It consists in putting the liquids into very flat 

 glass recipients, v hose two large lateral surfaces are perfectly parallel, 

 and the height four or five times that of the surface of the thermo- 

 electrical pUe. 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 



