RADIANT HEAT AND ITS RELATIONS. 229 



on chemical constitution, as influencing most powerfully 

 the phenomena of radiation and absorption. With regard 

 to gases, vapors, and to the liquids from which these va- 

 pors 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 ex- 

 tend this principle to solids I was met by a multitude of 

 facts obtained by celebrated experimenters, which seemed 

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

 the same radiant and absorbent power for chalk and lamp- 

 black. MM. Masson and Courtepee 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 where bodies are reduced to an extremely 

 fine state of division the influence of this state is so power- 

 ful as entirely to mask and override whatever influence 

 may be due to chemical constitution. 



But it appears to me that through the Avhole of these 

 researches a serious oversight has run, the mere mention 

 of which will show you what caution is essential in the 

 operations of experimental philosophy. Let me state 

 wherein I suppose this oversight to consist. I have here 

 a metal cube with two of its sides brightly polished. I 

 fill the cube with boiling water and determine the quan- 

 tity of heat emitted by the two bright surfaces. One of 

 them far transcends the other as a radiator of heat. Both 

 surfaces appear to be metallic ; what, then, is the cause of 

 the observed difference in their radiative power ? Simply 

 this : I have coated one of the surfaces with transparent 

 gum, through which, of course, is seen the metallic lustre 

 behind. Now this varnish, though so perfectly transparent 

 to luminous rays, is as opaque as pitch or lamp-black to 

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



