144 



WRITINGS OF JOSEPH HENRY. 



[1855- 



Melloni obtained by experimenting with different sources of 

 heat and different substances. 



Relative absorbing power by different substances of different kinds of heat. 



Substances. 



Lampblack 



White lead 



Isinglass 



Indian ink 



Shellac 



Polished metal 



Naked 

 flame. 



100 

 53 

 52 

 96 

 43 

 14 



Copper, at 

 212° F. 



100 

 100 

 91 

 85 

 72 

 13 



As an illustration of the effects of radiant heat of different 

 kinds, we may mention the fact, long observed, of the melt- 

 ing of snow near the trunks of trees and other dark-colored 

 bodies. That this effect is not due to the natural heat of the 

 plant is evident from the fact that it is equall}' exhibited 

 around the stumps of dead trees, and dark-colored objects of 

 an entirely different character. The rays of heat from the sun, 

 (as before stated,) possessing similar properties to those of 

 light, are absorbed by dark substances, and freely reflected 

 from light ones. The facets of the small crystals of snow re- 

 flect this heat almost entirely, while it is absorbed by the 

 dark surface of the wood of which it raises the temperature, 

 thus producing a new source of emanation. The heat how- 

 ever given off from the wood, is that of long waves of low 

 intensity, which is equally absorbed by light and dark 

 bodies; hence it enters the snow, raises its temperature, and 

 converts it from a solid to a liquid condition. We may im- 

 itate this action by supporting at a little distance above a 

 surface of new fallen snow a piece of pasteboard, both sides 

 of which have been covered with lampblack, and the whole 

 freely exposed to the sun's rays. It will be found that the 

 melting of the surface within the shadow is much more 

 rapid than that exposed to the direct rays of the sun. The 

 same result may be produced by the rays from an argand 

 lamp. Having filled a square box with new fallen snow 



