SECOND REPORT -1832. 
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ject 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 suf¬ 
ficed 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 nonluminous 
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, per¬ 
haps, 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 light. 
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 reflexion at an opake 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 ex¬ 
periments 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 different latitudes from the polar to the equatorial regions. 
