ON RADIANT HEAT. 67 



greatest avidity is ice or snow, which is merely ice in 

 powder. Hence, a less amount of heat will be lodged in 

 the cloth than in the surrounding snow. The cloth must 

 therefore act as a shield to the snow on which it rests; and, 

 in consequence of the more rapid fusion of the exposed 

 snow, its shield must, in due time, be left behind, perched 

 upon an eminence like a glacier-table. 



But though the snow transcends the cloth, both as a 

 radiator and absorber, it does not much transcend it* 

 Cloth is very powerful in both these respects. Let us no\v 

 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 influx 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 

 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 lampblack. MM. Massou and Courtcpee had per- 

 formed 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 con- 

 cluded from their researches, that when bodies are reduced 



