326 RADIANT HEAT. 



of his own, in which he has discovered the cause of all their discrep- 

 ancies. The position of the maximum heat in the spectrum depends 

 entirely on the nature of the medium employed — a circumstance 

 almost wholly unnoticed by former experimenters. 



The heating intensity is very small towards the violet extremity; 

 it thence gradually increases in prisms of water, -alcohol, or oil of tur- 

 pentine; the maximum is in the yellow space: in those of solution of 

 sal-ammoniac and corrosive sublimate, or sulphuric acid, it is in the 

 orange; in crown glass and common white 'glass, in the middle of the 

 red; in those glasses which contain much lead, it is in the limit of the 

 red; and in flint glass, beyond the visible boundary, but nearer to it 

 with Bohemian than with English glass. In all cases it gradually 

 diminishes from the maximum, and is perceptible to some distance 

 beyond the visible boundary. — (Schweigger's JSfeues. Journ., x, 129. 

 Annals of Phil., Sept., 1824; Abhandl. der Konigh. Acad. Wissensch of- 

 ten in Berlin, 1818-'19, p. 305; Phil. Mag., Nov. and Dec, 1825; 

 Edinb. Journ. of Science, No. II, 358.) 



4.) Analysis of the solar rays by the absorption of media. 



In respect to light, the remarkable variety in the absorption of dif- 

 ferent rays exhibited by different media has been well established, 

 and affords a new sort of analysis of light. 



In regard to the solar heat, similar researches have been made, 

 though as yet to little extent. The first observations of the kind 

 were those of Sir W. Herschel (Phil Trans., 1800.) He found the 

 absorption of several kinds of glass for his invisible rays and for the 

 middle red to be proportional to the following numbers out of 1,000 

 rays incident: 



Invisible rays. Red rays. 



Flint glass 000 143 



Coach glass 143 200 



Crown -lass 182 ••• • 294 



Dark red glass 000 G92 



5.) Sir D. Brewster has lately been engaged in some researches on 

 this subject, an abstract of which he has kindly communicated in 

 manuscript for this report. Agreeably to the view he has established 

 of the solar prismatic spectrum as consisting of spectra of three primary 

 colors superposed, and having their maxima at different points, he re- 

 gards the heating power as due, in like manner, to another primary 

 spectrum superposed in the same way, and similarly the chemical rays. 

 He makes the following statements with respect to the heating rays: 



1st. There is no proof whatever of the existence of invisible rays 

 of any kind beyond the red or the blue extremity of the spectrum. 

 Sir W. Herschel' s experiments prove the existence of heat beyond 

 the visible extremity of the spectrum which he used, but Sir D. Brewster 

 has succeeded in rendering the spectrum visible at every point where 

 any heat was produced. 



By particular processes he has traced the light at that end greatly 

 beyond the place where Frauenhofer makes the spectrum terminate. 



