324 RAW ANT HEAT. 



2.) I tried the point by two thermometers (as in the case of terres- 

 trial heat) and found no perceptible difference in the ratio, with and 

 without the screen, of the black and white thermometers. — (Annals of 

 Phil, xli, 321.) 



The same result was found with a differential thermometer, with a 

 glass screen over the bulb, which was not blackened. No difference 

 was observable between the indication under these circumstances, 

 and when both were exposed. — (Annals of Phil., xlii, 401.) 



Hence, I think we are entitled to conclude that there does not exist 

 in the solar beam, in its natural state, any simple radiant heat, (as 

 before defined;) but that the whole emanation consists of the other 

 species, distinguished by the two characteristics of affecting sub- 

 stances with heat in proportion to the darkness of their color, and 

 being wholly transmissible through glass without heating it, and in- 

 separable from the rays of light. 



This applies to the rays of the sun which come within the reach of 

 our examination. It must, however, be admitted, as by no means 

 improbable, that the sun may originally give out a separate radiation 

 of simple heat. None of this kind reaches us; but Ave must consider 

 the very different degree in which any medium, as air, absorbs or 

 intercepts the passage of those two sorts of radiant agents. The heat 

 from a hot body will not be perceptible at a short distance, while its 

 light will traverse an amazing extent of length; and thus at different 

 distances the ratio between the two sorts of heating effect will be 

 very different. Some degree of simple heat, therefore, may actually 

 be initially radiated by the sun and be lost before it reaches us. We 

 do not know that there is any medium between the different parts of 

 the solar system capable of absorbing heat. The highest regions of 

 our atmosphere into which observation has penetrated are uniformly 

 the coldest; but they are known to have a greater capacity for heat. 

 Thus, though it is possible that some heat may reach to that distance 

 and be absorbed without becoming sensible to us, its quantity must 

 be very small; if, therefore, we suppose any simple heat to be initially 

 radiated from the sun, it must be all, or nearly all, absorbed by some 

 parts or appendages of that luminary exterior to the part where it 

 is generated. 



3.) The concentration of the sun's heat by a lens is a familiar experi- 

 ment. 

 ^ Sir W. Herschel (Phil. Trans., 1800, Exp. 23) concludes that there 

 ^is a focus of greatest heat farther from the lens than that of light; 

 sealing-wax was scorched in the same time when in the luminous 

 focus, and at half an inch further from the lens; this affords no proof 

 of its being separated from the light. 



That the heat is found to accompany the rays of light in the most 

 constant and inseparable manner through various refractions, as in 

 the instance of the four lenses in the eye-piece of a telescope after 

 reflection, is also remarked by Sir W. Herschel, (Phil. Trans., 1800, 

 Exp. 11.) 



