48 DISTRIBUTION OP HEAT. 



the chemical agencies of the solar beam, our attention is first to be directed to its cal- 

 orific properties. The following facts, which are gathered from the writings of M. 

 MELLONI, who has cultivated this department of physical science with such remarka- 

 ble success, will serve to give an idea of the opinions which have been advocated by 

 that eminent philosopher. 



165. The different illuminating power of the different regions of the spectrum is, of 

 course, plainly evident to the eye. NEWTON supposed that the calorific effect of the 

 different spectrum regions followed the same order, and, therefore, that the yellow space 

 was the hottest. In this view he was followed by LANDRIANI, SENNEBIER, ROCHON, 

 and others. But at a later period, Sir W. HERSCHEL having observed that when dif- 

 ferent coloured pieces of glass were employed to intercept the solar rays, the amount 

 of heat which traversed them was not in proportion to the amount of light, he made 

 trials on the solar spectrum by putting thermometers in its different colours and mark- 

 ing their elevations. In the course of these experiments he substantiated two remark- 

 able facts : 1st. That below the red region, and in a space where no light could be 

 seen, the thermometer rose. 2d. That the point of maximum temperature was not, as 

 NEWTON supposed, in the yellow, but totally out of the coloured spaces, and among these 

 dark rays which are less refrangible than the red rays of light. 



166. These experiments were repeated by various chemists, and with very contra- 

 dictory results, some affirming the results of Sir W. HERSCHEL, and others dissenting 

 from them. SEEBECK, however, eventually proved that this arose from the nature of 

 the prism used. That when flint glass was employed, the maximum point was in the 

 dark rays ; that when crown glass was used, it was at the extremity of the red ; for sul- 

 phuric acid and alcohol, it was in the orange ; for water, it was in the yellow. 



167. M. MELLONI repeated these experiments with the aid of an improved thermom- 

 eter, and established their correctness. He took a prism of crown glass, which gave the 

 maximum of temperature at the extremity of the red, and determined the distribution 

 of heat in the coloured spaces, and also in the dark region outside of the red. Then 

 he interposed a layer of water, so that the rays emerging from the prism were trans- 

 mitted through it, and again measured the temperatures in the obscure and the lumi- 

 nous spaces. They were found altered ; some were totally destroyed, others partially, 

 and others had escaped without sensible change. The greatest absorption had taken 

 place among the least refrangible, the action having been less and less as the red ray 

 was approached ; it diminished through the red, the orange, and a part of the yellow, 

 and then to the end of the violet it disappeared. 



168. The direct effect of this partial absorption is, of course, to move the maximum 

 point, and bring it successively through the red, the orange, and the yellow region. These 

 experiments also show that the heat distributed through the solar spectrum is not a ho- 

 mogeneous agent, for the same medium acts differently on its differently refrangible parts. 

 In the same manner, M. MELLONI showed that the calorific rays arising from artificial 

 sources, whether luminous or dark, were heterogeneous, and perfectly like the analo- 

 gous rays existing in the sunbeam. That while most transparent substances had the 

 quality of absorbing some of these rays, and allowing others to pass, there is one body, 



