290 
SECOND REPORT— -1832. 
with, and without the screen, of the black and white thermo¬ 
meters. (. 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 substances with heat in proportion to the darkness 
of their colour, and being wholly transmissible through glass 
without heating it; and inseparable 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 we 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 dif¬ 
ferent. 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 hy a lens is a familiar 
experiment. 
Sir W. Herschel {Phil. Trans. 1800, Exp. 23,) concludes that 
there is a focus of greatest heat further 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. 
