177 
game substance, a little blackened, was exposed to 
red light, it recovered in part its whiteness, especial- 
ly when presented to the invisible calorific r,ys, 
which lie beyond the limit of the red extremity. 
He observed, also, that phosphorus, when placed 
near the red rays, instantly exhibited white vapours ; 
but, when moved into the violet rays, these vapours 
no longer appeared, and its combustion was extin- 
guished. Hence he was led to conclude, that the 
solar spectrum was comprised between two sets of 
invisible rays, which produce the opposite effects of 
combining and separating oxygen from bodies, while 
the intermediate parts of the spectrum partake more 
or less of the action that is exerted by the invisible 
rays on either side *. These chemical rays were, 
likewise^ discovered by Dr Wollastonj who found 
that muriate of silver was blackened, not only in the 
space occupied by the violet ray, but in an equal de- 
gree, and to about an equal distance, beyond the vi- 
sible spectrum ; and, by narrowing the pencil of 
light received on the prism, the discolouring rays 
were made to fall almost entirely beyond the violet : 
whence he inferred, that visible light does not pos- 
sess this discolouring property, but owes its in- 
fluence, in this respect, to the admixture of invisible 
light f. 
443. From the foregoing series of facts, it would 
appear, that solar light is made up of three distinct 
species of rays ; one species of which produces 
heat, and promotes the combination of oxygen with 
* Haiiy, vol. ii. p* 259- 1 PM- Trans. 1S02, p. 37p, 
