6^ Dr. Herschel on the Nature and Construction 
This may be very substantially answered by many proofs 
drawn from natural philosophy, which shew that heat is pro- 
duced by the sun's rays only when they act upon a calorific 
medium; they are the cause of the production of heat, by 
uniting with the matter of fire, which is contained in the sub- 
stances that are heated : as the collision of flint and steel will 
inflame a magazine of gunpowder, by putting all the latent 
fire it contains into action. But an instance or two of the 
manner in which the solar rays produce their effect, will bring 
this home to our most common experience. 
On the tops of mountains of a sufficient height, at an alti- 
tude where clouds can very seldom reach, to shelter them from 
the direct rays of the sun, we always find regions of ice and 
snow. Now if the solar rays themselves conveyed all the heat 
we find on this globe, it ought to be hottest where their course 
is least interrupted. Again, our aeronauts all confirm the 
coldness of the upper regions of the atmosphere; and since, 
therefore, even on our earth the heat of any situation depends 
upon the aptness of the medium to yield to the impression of 
the solar rays,, we have only to admit, that on the sun itself, 
the elastic fluids composing its atmosphere, and the matter on 
its surface, are of such a nature as not to be capable of any ex- 
cessive affection from its own rays ; and, indeed, this seems to 
be proved by the copious emission of them ; for if the elastic 
fluids of the atmosphere, or the matter contained on the sur- 
face of the sun, were of such a nature as to admit of an easy, 
chemical combination with its rays, their emission w'ould be 
much impeded. 
Another well known fact is, that the solar focus of the 
largest lens, thrown into the air, will occasion no sensible 
heat in the place where it has been kept for a considerable 
