RADIANT HEAT AND ITS RELATIONS 97 



Cloth is very powerful in both these respects. Let us 

 now turn our attention to the piece of black cloth, the 

 texture and fabric of which I assume to be the same as 

 that of the white. For our object being to compare the 

 effects of color, we must, in order to study this effect in 

 its purity, preserve all the other conditions constant. Let 

 us then suppose the black cloth to be obtained from the 

 dyeing of the white. The cloth itself, without reference to 

 the dye, is nearly as good an absorber of heat as the snow 

 around it. But to the absorption of the dark solar rays 

 by the undyed cloth is now added the absorption of the 

 whole of the luminous rays, and this great additional in- 

 flux of heat is far more than sufficient to turn the balance 

 in favor of the black cloth. The sum of its actions on the 

 dark and luminous rays exceeds the action of the snow 

 on the dark rays alone. Hence the cloth will sink in the 

 snow, and this is the complete analysis of Franklin's 

 experiment. 



Throughout this discourse the main stress has been laid 

 on chemical constitution, as influencing most powerfully 

 the phenomena of radiation and absorption. With regard 

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

 vapors 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 an extension. Melloni, for example, 

 had found the same radiant and absorbent power for chalk 

 and lamp-black. MM. Masson and Courtdpee had per- 

 formed a most elaborate series of experiments on chemi- 



SCIENCE V 5 



