90 FRAGMENTS OF SCIENCE.' 



petent to absorb the luminous rays. Whether, then, 

 the cloth will sink or not depends entirely upon the 

 dark rays of the sun. Now the substance which absorbs 

 these dark rays with the 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 colour, 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 favour 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. 



