322 Mr. Powell on Solar Light and Heat. [May, 



heat. Light is transmitted in a way which we term radiation. 

 The heat from non-luminous hot bodies is transmitted to a dist- 

 ance 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. 



(2.) 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 existed a separate set of 

 heating 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 property 

 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. 



Among the earliest experiments on the subject, if not actually 

 the first, were those of Mr. Boyle, on the different degrees of 

 heat communicated by the sun to black, white, and red-coloured 

 surfaces. These were extended and confirmed in the well- 

 known investigations of Dr. Franklin, &c. 



" Mr. Boyle caused a large block of black marble to be 

 ground into the form of a spherical concave speculum, and found 

 that the sun's rays reflected from it were far from being too 

 powerful for his eyes, as would have been the case had it been 

 of any other colour; and although its size was considerable, yet 

 he could not set a piece of wood on fire with it; whereas a far 

 less speculum of the same form, made out of a more reflecting 

 substance, would presently have made it flame." — (Boyle on 

 Colours, 8cc.) 



Scheele conceived that the sun's rays of light produced heat, 

 not when in motion, but only when stopped by the interposition of 

 solid bodies. — (Treatise on Air and Fire, &c.) 



Mr. Melville seems to have viewed the matter nearly in the 

 same light, and to have conceived reflexion at an opaque surface, 

 the cause of excitation of heat from the sun's rays. — (See Phil. 

 Mag. June, 1815, a paper by Dr. Evans.) 



(3.) In later times the experiments of Prof. Leslie, Sir H. 

 Davy, 8cc. have sufficiently established the property possessed 

 by that emanation (whatever its nature may be, whether simple 

 or compound) which is derived from the sun, of producing 

 greater heat in bodies in proportion as their surfaces owing to 

 darkness of colour, have the capacity of absorbing rays of light. 



