CONTRIBUTIONS TO MOLECULAR PHYSICS 419 



changing anything but the temperature of the source; its 

 size, distance, and surroundings remaining the same. The 

 experiments proved rock-salt to be colored thermally. It 

 is more opaque, for example, to the radiation from a barely 

 visible spiral than to that from a white-hot one. 



In regard to the relation of radiation to conduction, if 

 we define radiation, internal as well as external, as the 

 communication of motion from the vibrating atoms to the 

 ether, we may, I think, by fair theoretic reasoning, reach 

 the conclusion that the best radiators ought to prove the 

 worst conductors. A broad consideration of the subject 

 shows at once the general harmony of this conclusion with 

 observed facts. Organic substances are all excellent radia- 

 tors; they are also extremely bad conductors. The mo- 

 ment we pass from the metals to their compounds we pass 

 from good conductors to bad ones, and from bad radiators 

 to good ones. Water, among liquids, is probably the 

 worst conductor; it is the best radiator. Silver, among 

 solids, is the best conductor; it is the worst radiator. The 

 excellent researches of MM. de la Provostaye and Desains 

 furnish a striking illustration of what I am inclined to 

 regard as a natural law — that those atoms which transfer 

 the greatest amount of motion to the ether, or, in other 

 words, radiate most powerfully, are the least competent 

 to communicate motion to each other, or, in other words, 

 to propagate by conduction readily. 



