RADIANT 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 me- 

 chanical, condition. In attempting to extend this prin- 

 ciple to solids I was met by a multitude of facts, ob- 

 tained 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 radia- 

 tion. They concluded from their researches, that when 

 bodies are reduced to an extremely fine state of di- 

 vision, the influence of this state is so powerful as en- 

 tirely 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 lumi- 

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

 luminous ones. It is a powerful emitter of dark rays; 



