Experiments and Observations on the Solar Rays. 29 
fects of the sun’s rays upon them. Philosophers explain this 
difference, by supposing that white bodies reflect the greater part 
of the incident rays, and black bodies absorb them all or nearly 
all. ‘In black bodies, such as coal, &c. all the rays which enter 
their substance are absorbed, and hence we see the reason why 
such bodies are more easily heated and inflamed by the action of 
the luminous rays.”* Sir Davin Brewster here speaks as if the 
visible rays contribute as much to the increase of heat as the ca- 
lorific rays, and also, as if both are acted upon by the coal, &e. 
in the same manner. But it is certain that the coal acts very dif- 
ferently on the former from what it does on the latter. ‘The mo- 
ment the rays fall on the coal, the white light which enters its 
substance disappears—it completely escapes the cognizance of 
the senses; but the caloric exists within the pores of the coal im a 
free state, and then escapes in a sensible manner. 
It was the opinion of Sir Isaac Newron, that the light which 
passes into the substance of an opaque body, is “ variously reflect- 
ed to and fro within the body till it happens to be stifled and 
lost.” Later philosophers assert that the particles of bodies ex- 
ercise a specific attraction over the particles of light.. ‘“ Whena 
portion of light,” says the celebrated Sir Davin Brewster, “en- 
ters a body and is never again seen, we are entitled to say that it 
is detained by some power exerted over the light by the particles 
of the body. That it is attracted by the particles seems extreme- 
ly probable, and that it enters into combination with them and 
produces various chemical and physical effects, cannot well be 
doubted.”+ Now, as when the sun’s rays fall upon coal or any 
black body, the greater part of the rays enter the substance of the 
body, as the intromitted white light is never again seen; but as 
the caloric exists in a free state, there is every reason to believe 
that the former, but not the latter, enters into combination with 
the body. The circumstance of the caloric’s quickly effecting 
its escape from the internal parts of the body, by radiation and 
otherwise, affords positive proof that it is not acted upon by that 
specifie attraction which, in all probability, detains the white 
light ; nevertheless, it must be acted upon slightly by an attrac- 
tive force of another kind, or else its total discharge from the in- 
* Brewster's Opt., chap. xxxiv. — t Opt., chap. xxxrv. 
