RADIANT HEAT. 319 



nous hot bodies is transmitted to a distance in a way closely analogous, 

 and to which the same name has been applied. 



In the first instance, we might suppose that the sun sends out two 

 separate emanations — one of light, and another distinct from it, and 

 similar to that of radiant heat from a mass of hot water; and this, 

 perhaps, was the first view taken of the subject, though a confused 

 idea of some very close and intimate connexion subsisting between 

 the solar light and heat appears to have prevailed. 



This subject, as might naturally be expected, attracted the early 

 notice of experimenters. A very slight examination sufficed to show 

 that the rays of solar heat, (whatever their nature might be,) differed 

 essentially in many properties from those of terrestrial heat, whether 

 radiated from luminous or non-luminous bodies. Whether there ex- 

 isted a separate set of healing rays distinct from those of light, and 

 at the same time differing in many respects from rays of terrestrial 

 heat; or whether these differences depended on some unknown prop- 

 erty of the rays of light, was a question which for a long time remained 

 without any direct investigation, and on which even now we have, 

 perhaps, no very precise ideas. 



I. — Solar rays in their natural state. 



a.) Nature of radiation. 



1.) The solar heat is transmitted through the air without heating it. 



It invariably accompanies the l^'ht. 



Scheele conceived that the sun's rays of light produced heat not 

 when in motion but when stopped by the interposition of solid bodies. 

 — (On Air and Fire, &c.) 



Mr. Melville seems to have adopted nearly the same theory, and to 

 have conceived reflection at an opaque surface to be the cause of an 

 excitation of heat from the sun's rays. — (Evans on the Calorific Rays, 

 &c, Phil. Mag., June, 1815.) 



In general, for light of the same composition the heat appears 

 nearly proportional to the illuminating intensity. 



2.) Measures of radiation. 



Theory of the sensibility of thermometers especially for experiments 

 of this kind.— (Sir W. Herschel, Phil. Trans., 1800, Note, p. 447.) 



Leslie contends for the exact proportionality of intensity of light 

 and heating power. — (Inquiry, pp. 160 and 408.) 



Theory and construction of his "Photometer," ch. xix, p. 403. 



Ritchie's "Photometer," of the same kind. — (Phil. Trans., 1825, 

 Part I, p. 141.) See his Remarks on Leslie's Photometer, Edinb. 

 Journ. of Science, No. IV, 321, and V, 104. 



Mr. Daniell, in his work on Meteorology, has collected a great 

 number of observations on the heating power of the sun's rays in dif- 

 ferent latitudes from the polar to the equatorial regions. Most of 

 these observations were made by comparing two thermometers, one 

 of which was kept in the shade, whilst the other, having its bulb 



