RADIANT HEAT. 321 



especially in windy weather. To avoid an inconveniently long scale, 

 there should be two instruments constructed, one for winter and the 

 other for summer. The Professor has kept for nearly a year a regis- 

 ter of sunshine. 



b.) Reflection of solar heat. 



1.) It takes place exactly by the same laws as that of the light. 



The heat is collected in the focus of concave reflectors along with 

 the light. 



2.) The sun's rays reflected from the moon are probably much too 

 feeble to allow of any heat being made sensible. 



Dr. Howard, however, states that, with a peculiar differential ther- 

 mometer, he has obtained an effect. — (Silliman's American Journal,vol. 

 ii, 329.) 



MM. Melloni and Nobili (with the apparatus before described) tried 

 to detect heat in the moon's rays, but without success; they mention, 

 however, that terrestrial radiation interferes greatly with such exper- 

 iments, and do not describe fully their contrivances for obviating this 

 cause of error. — (Ann. de Chimie, Oct., 1831, p. 210.) 



3.) Berard (memoir before cited) tried the 'polarization of the solar 

 heat; that is, polarized the sun's light; and in the position of non- 

 reflection found that the heat had disappeared with it. — (See Edinb. 

 Journ. of Science, vi, 297.) 



c.) Under this head nothing known. 



d.) Effect of surface on the absorption of solar heat. 



1.) I am not aware of any experiments directly showing how far the 

 same relation to the texture of surfaces which has been found in ab- 

 sorption of simple heat may hold good in regard to the sun's rays. 

 But for surfaces of the same texture it has been incontrovertibly estab- 

 lished that the effect in this case increases in proportion to the dark- 

 nets of color, or in proportion to the absorption of light; and it would 

 seem most probable that this relation is the only one which really 

 holds good, the texture of the surface being probably quite indifferent 

 except so far as it tends to the better absorption of the light. 



2.) 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 colored surfaces. 



He 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 color; 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 inflame. 



It was remarked by Scheele that the thermometer, when filled with 

 alcohol of a deep red color, rose more rapidly when exposed to the 

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