CONTRIBUTIONS TO MOLECULAR PHYSICS. 397 



It is worthy of remark that whenever, through a 

 change of source, the position of a vapour as an ab- 

 sorher of radiant heat was altered, the position of the 

 liquid from which the vapour was derived underwent a 

 similar change. 



It is still a point of difference between eminent 

 investigators whether radiant heat, up to a temperature 

 of 100 C., is monochromatic or not. Some affirm this ; 

 some deny it. A long series of experiments enables 

 me to state that probably no two substances at a tem- 

 perature of 100 C. emit heat of the same quality. 

 The heat emitted by isinglass, for example, is different 

 from that emitted by lampblack, and the heat emitted 

 by cloth, or paper, differs from both. It is also a subject 

 of discussion whether rock-salt is equally diathermic to 

 all kinds of calorific rays ; the differences affirmed to 

 exist by some investigators being ascribed by others to 

 differences of incidence from the various sources em- 

 ployed. MM. de la Provostaye and Desains maintain 

 the former view, Melloni and M. Knoblauch maintain 

 the latter. I tested this point without changing any- 

 thing but the temperature of the source ; its size, 

 distance, and surroundings remaining the same. The 

 experiments proved rock-salt to be coloured 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 reason- 

 ing, 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 radiators ; they are also extremely bad 



