244 M. Melloni on the Cause of the speedy Melting 



less in intensity than the solar heat itself. But, if the events 

 occur as is usually supposed, the very contrary of what we ob- 

 serve should happen, so that in open places, where only the shor- 

 dozes projected by trees and bushes are thrown, the snow should 

 disappear more speedily than in spots completely overshadowed 

 hy plants ; and we should no longer have the scientific discre- 

 pancy of observing the greatest effect where the cause is least. 

 The explanation, therefore, adds M. Fusinieri, of these facts, 

 by the ordinary theory of radiated caloric, cannot be admitted. 



I allow that the melting of snow under the action of radiat- 

 ing caloric, ought to increase in proportion to the energy of the 

 incident rays : I also allow that the direct heat of the sun ought 

 greatly to surpass in intensity the heat which emanates from 

 branches and trunks of trees, which are only heated by its in- 

 fluence. But in maintaining that, in the observed phenomena, 

 the effect is, so to speak, in the inverse ratio of the cause, it 

 would be previously necessary to prove that snow with equal 

 facility absorbs the direct solar rays, and those which are emit- 

 ted by the heated bodies of plants. Else, if these latter rays 

 are much more readily absorbed than the former, there will be 

 no manner of contradiction, and the less action of the more in- 

 tense rays will be only a natural consequence of their less ready 

 absorption. The error of M. Fusinieri arises from this circum- 

 stance that he still admits, with Leslie and Rumford, a unifor- 

 mity in the absorbing power of bodies for all kinds of radiating 

 heats, whilst our experiments have demonstrated that these 

 powers are liable to very great changes, w^hen we vary the qua- 

 lity of the calorific rays. 



That we might produce a fact every way analogous to that 

 now before us, I freed my thermo-electrical pile of the lamp- 

 black which usually covers it ; I then painted it white with 

 carbonate of lead, and, after having supplied it with its small 

 tubes, I shut one side, and caused the light of a lamp, concen- 

 trated by a lens, to fall upon the other. The galvanometer, 

 when put into communication with the pile, presently marked a 

 constant deviation of 15°. After this I interposed on the pas- 

 sage of the rays, and quite near to the pile, a sheet of thick 

 paper, of a deep grey colour, and speedily the deviation of the 

 galvanometer was increased, and after a few minutes terminated 



