ON RADIANT HE A T. 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 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 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. Melloui, for example, 
had "fount] the same radiant and absorbent power for chalk 
and lampblack. MM. Masson and Courtepee 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 
