and the Mode of its Communication. 99 



the confines of the medium, or to the surface of some 

 other body. 



If this hypothesis of the communication, or rather 

 generation, of heat and of cold by radiation be true, 

 it will enable us to explain, in a satisfactory manner, 

 what has been called the non-conducting power of trans- 

 parent fluids with respect to heat ; for, if heat be real- 

 ly communicated or excited in the manner above de- 

 scribed, it is quite evident that a perfectly transparent 

 fluid can receive heat only at its surface, and, conse- 

 quently, that heat cannot be propagated in such a fluid 

 by communication from one particle of the fluid to an- 

 other. 



By a transparent fluid I mean such an one as admits 

 the calorific and frigorific rays emitted by hot and by 

 cold bodies to pass freely through it without obstruct- 

 ing their passage or diminishing their intensities. 



Whether any of the fluids with which we are ac- 

 quainted be perfectly transparent in this sense of the 

 word or not, I will not pretend to say ; but there is 

 reason to think that pure water and air and most other 

 fluids which are transparent to light, possess a high de- 

 gree of transparency in regard to calorific and frigorific 

 rays, or that they give a very free passage to them when 

 they have once passed their surfaces. 



An even or polished surface has been found to facili- 

 tate very much the reflection of the rays of light. May 

 it not, in all cases, have an equal tendency to facilitate 

 the reflection of calorific and frigorific rays ? 



In the experiments with the large cylindrical vessels, 

 where they were exposed naked to cool in the air, their 

 surfaces were polished, and they were a long time in 

 cooling. But, when the surface of the vessel was black- 



