and the Mode of its Communication. 151 
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 transparent fluids, with respect to heat ; 
for, if heat be really communicated, or excited, in the manner 
above described, it is quite evident that a perfectly transparent fluid 
can receive heat only at its surface ; and consequently, that heat 
cannot be propagated in such a fluid, by communication, from 
one particle of the fluid to another. 
By a transparent fluid, I mean such an one as admits the ca- 
lorific and frigorific rays, emitted by hot and by cold bodies, to 
pass freely through it, without obstructing their passage, or di- 
minishing their intensities. 
Whether any of the fluids with which we are acquainted 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 degree 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 facilitate 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 blackened, or covered with other sub- 
stances, the vessel was found to cool much more rapidly. 
A large proportion of the frigorific rays from the surrounding 
