CONTRIBUTIONS TO MOLECULAB PHYSICS. 397 



tion from the isinglass most powerfully; the single 

 exception was chloroform. 



It is worthy of remark that whenever, through a 

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

 sorber 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 ., 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 sub- 

 ject of discussion whether rock-salt is equally dia- 

 thermic 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 employed. MM. de la Provostaye and Desains 

 maintain the former view, Melloni and M. Knoblauch 

 maintain the latter. I tested this point without 

 changing anything but the temperature of the source; 

 its size, distance, and surroundings remaining the same. 

 The experiments proved rock-salt to be coloured ther- 

 mally. 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 



