176 
ter, placed in its focus, only one degree; while 
the red rays, in two minutes and a half, raised it 
sixteen degrees ; and when the thermometer was 
carried quite out of visible light, it rose, in two 
minutes and a half, eighteen degrees *. From these 
experiments we learn, that the heating power of solar 
light consists in invisible rays, which are entirely dis- 
tinct from those which produce illumination and 
colour. It may be added, that Scheele had previous- 
ly ascertained, that the caloric, which radiates from 
a common fire, and causes ignition, consists, likewise, 
in invisible rays f. 
442. But we have also seen (432.), that the che- 
mical action of light is most powerfully exerted by 
the violet rays, which are far removed from the 
centre of the spectrum, where the illuminating power 
is greatest, and still farther from its red extremity, 
where the calorific effect is most intense. As, there- 
fore, in their refrangibility, they seem to follow laws 
so distinct from the two other species, it was natural 
to expect that they should, also, consist of a distinct 
kind, or third species of matter. This, M. Ritter 
has discovered to be actually the case, and that, like 
the calorific rays, these chemical rays are invisible, 
and possess the greatest power beyond the violet 
boundary of the spectrum. He found that muriate 
of silver, which became black when placed beyond 
the confines of the violet ray, gradually lost its dark 
tint, as it was moved through the other rays towards 
the red extremity of the spectrum ; and when the 
* Murray's Syst. Chem. vol. i. p. 519. t On Air and Fire. p. 76*. 
