KADIANT HEAT AND ITS RELATIONS. 91 



With regard to gases and vapours, and to the liquids 

 from which these vapours are derived, it has been 

 proved by the most varied and conclusive experiments 

 that the acts of radiation and absorption are molecular 

 that they depend upon chemical, and not upon 

 mechanical, condition. In attempting to extend this 

 principle to solids I was met by a multitude of facts, 

 obtained by celebrated experimenters, which seemed 

 flatly to forbid such an extension. Melloni, for example, 

 had found the same radiant and absorbent power for 

 chalk and lamp-black. MM. Masson and Courtepee 

 had performed a most elaborate series of experiments on 

 chemical precipitates of various kinds, and found that 

 they one and all manifested the same power of radiation. 

 They concluded from their researches, that when bodies 

 are reduced to an extremely fine state of division, the 

 influence of this state is so powerful as entirely to mask 

 and override whatever influence may. be due to chemical 

 constitution. 



But it appears to me that through the whole of these 

 researches an oversight has run, the mere mention of 

 which will show what caution is essential in the opera- 

 tions of experimental philosophy ; while an experiment 

 or two will make clear wherein the oversight consists. 

 Filling a brightly polished metal cube with boiling 

 water, I determine the quantity of heat emitted by two 

 of the bright surfaces. As a radiator of heat one of 

 them far transcends the other. Both surfaces appear to 

 be metallic ; what, then, is the cause of the observed 

 difference in their radiative power ? Simply this : one 

 of the surfaces is coated with transparent gum, through 

 which, of course, is seen the metallic lustre behind ; and 

 this varnish, though so perfectly transparent to luminous 

 rays, is as opaque as pitch, or lamp-black, to non-lumi- 

 nous ones. It is a powerful emitter of dark ra} r s ; it 



