228 FRAGMENTS OF SCIENCE. 



powerless; the one cannot be warmed, nor the other 

 melted, by such rays. The cloth is white and the snow is 

 white, because their confusedly mingled particles and fibres 

 are incompetent to absorb luminous rays. Whether, then, 

 the cloth will sink or not depends entirely upon the dark 

 rays of the sun. Now the substance which absorbs the 

 dark rays of the sun with the greatest avidity is ice — or 

 snow, which is merely ice in powder. A less amount of 

 heat will be lodged in the cloth than in the surrounding 

 snoAV. 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, the cloth must in due 

 time be left behind, perched upon an eminence like a gla- 

 cier-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 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 siioav 

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

 snow, and this is the philosophy of Franklin's experi- 

 ment. 



Throughout this discourse the main stress has been laid 



