RADIANT HEAT. 325 



II. — Solar rays subjected to analysis by the prism. 



1.) The different heating powers belonging to different parts of the 

 spectrum were probably first observed by the Abbe Rochon. — {Phil. 

 Mag., June, 1815; and Biot, Traiie dePhys., iv. , GOO.) He found the 

 maximum in the yellow-orange rays: tho prism was of flint glass: his 

 thermometer was filled with spirits, probably therefore tinged red; 

 this may account for his result. 



1 tried some experiments- with the bulb of the thermometer painted 

 red, which appeared to agree with his result. — {Annals of Phil., li, 

 201.) 



Professor Leslie applied his "photometer " to these experiments. — 

 {Inquiry, p. 454.) 



Dr. Hutton observed the different heating powers, and that they 

 are not proportional to the illuminating. — {Diss, on Light and Heat, 

 p. 38.) _ 



Landriani found the maximum in the yellow rays, as also did Sene- 

 bier. — (Volta, Letter e, &c, 130.) 



Berard {Mem d? Arcueil, iii; Ann. de Chimie, lxxxv, 309) repeated 

 the experiment with a heliostat. He found the maximum in the red, 

 but some heat beyond. He repeated the experiment in both the 

 spectra formed by Iceland spar. 



2.) Sir W. Herschel {Phil. Trans., 1800, Part II) first observed the 

 maximum of heat beyond the red end of the visible spectrum, and 

 considered the effect as due to essentially invisible rays of a separate 

 kind from those of light. 



Yet he found them subject to the same laws of refraction, and 

 their dispersion corrected by another prism: they were concentrated 

 by a lens {Ibid., p. 317.) and by reflection (pp. 298, 302.) 



Leslie objects to the conclusion of invisible rays, and tries to 

 account for it as owing to an optical cause. — {Inquiry, Note, p. 559; 

 see also Nicholson 1 s Journal, 4to, iv, 344 and 410.) 



Sir H. Englefield {Nicholson's Journal, iii, 125,) found heat beyond 

 the visible red; it does not appear whether it was there at a maxi- 

 mum: the rays w r ere such as to be concentrated by a lens, and he 

 compared the effects on a black and white bulb. The exterior effect 

 on the white bulb was in a much l^ss ratio to that within the visible 

 spectrum than on the black. 



Sir H. Davy repeated these experiments in the clear atmosphere 

 of Italy, and with thermometers of extremely minute size, to secure 

 an instantaneous effect: he found the maximum beyond the red. 



These experiments, were also tried by Ritter and by Professor 

 Wiinsch {Magazin der Gesellsch, &c, Berlin, 1807.) He used prisms 

 of different substances; with alcohol, oil of turpentine, and water, the 

 maximum was in the yellow; with green glass in the red; and with 

 yellow glass on the extreme boundary. 



3.) But by far the most important and conclusive researches on this 

 subject are those of Dr. Seebeck, who in a memoir read to the Royal 

 Academy of Berlin, after discussing the conclusions and views of pre- 

 vious experimenters, proceeds to an elaborate series of experiments 



