OF ARTS AND SCIENCES. 145 



the radiation of heat, in which his measurements were made with the 

 thermopile instead of mercury thermometers. 



These researches included a study, made with this instrument, of the 

 distribution of heat in the spectrum. He found, as Wunsch and 

 Seebeck had done, that the distribution of heat was dependent upon 

 the material of the prism used, and that the reason why different 

 substances gave different spectra was because each possessed an ab- 

 sorptive power for certain of the heat rays. The selective absorption 

 of some substances gave rise to sinuosities in the curve of distribution. 



There was one substance, however, that he found to be transparent 

 for all rays, both of light and heat. This was rock-salt. Fluor-spar 

 he found to be the nearest to approach it in diathermacy. Resuming 

 the study of the spectrum with prisms of rock-salt and the thermopile, 

 he found the maximum to be at a distance beyond the red equal to 

 the distance of the yellow from the same. 



In the Philosophical Transactions for 1840, Sir John Herschel 

 published the results of a series of measurements of the distribution 

 of heat in the spectrum, made by exposing to its rays a thin black- 

 ened paper, which had been covered with some easily evaporated 

 liquid. The points of greatest intensity of heat showed themselves 

 by the greatest evaporation of the liquid. His results were, in gen- 

 eral, similar to those previously obtained, but, in addition, he claimed 

 to have discovered lines of discontinuity, similar to the Fraunhofer 

 lines of the luminous spectrum. 



Melloni, in the Comptes Rendus * for the same year, criticises the 

 results obtained by Herschel, and says that they cannot be regarded 

 as an accurate measurement of the distribution of heat, because the 

 blackened paper would not absorb all the rays in the same proportion, 

 and because its conductibility would extend the evaporation at points 

 of great to those of low intensity. The lines of discontinuity he 

 claims to have discovered himself, and to have explained by the selec- 

 tive absorption of the prism. 



In 1858 a research upon the distribution of heat in the sun's 

 spectrum was made by J. Miiller.f He used prisms of rock-salt and 

 of glass for obtaining the spectra, and a carefully constructed ther- 

 mopile and galvanometer for estimating the temperatures. His 

 experiments included a study of the effects of interposing various 

 colored solutions in the paths of the rays, and, later, studies of the 

 effects of using different prisms. With regard to the position of the 



* T. xi. t Pogg. Ann., cv. 



vol. xiv. (n. s. vi ) 10 



