175 
visible, and enable them to exhibit colour, possess 
peculiar properties, and are entirely distinct from 
those which excite or produce heat. 
441. The heating or calorific rays, which enter 
into the composition of solar light, appear to have 
been first distinguished as a distinct species of mat- 
ter, by M. Rochon, who, in the year 1775, disco- 
vered that the rays, which differ in refrangibility, 
differ also in their power of heating bodies. He ob- 
served that an air thermometer, moved through the 
prismatic spectrum, rose in proportion as the rays 
followed one another from the violet to the red ex- 
tremity ; so that the heat between clear red and the 
most intense violet appeared nearly as eight to one *. 
M. Senebier also, (438.), by a similar mode of ex- 
periment, ascertained the same fact with less preci- 
sion j and Dr Herschell, in a similar manner, calcu- 
lated the difference nearly as seven to two. Pur- 
suing the subject still farther, Dr Herschell ascer- 
tained that this calorific effect was produced by rays, 
different from those which impart light and colour ; 
that it was, in fact, produced by invisible rays, which 
increase progressively in power from the violet to 
the red extremity of the spectrum, and exert the 
greatest effect about half an inch beyond the boun- 
dary of the latter f. These facts are farther confirmed 
by Sir H. Englefield, who having successively collect- 
ed the prismatic rays into the focus of a lens, found 
the violet, in three minutes, to raise the thermome- 
* Huiiy's Nat. Phil. vol. ii. j>. ? jC. 
1 Phil. Trans. 1 SCO. 
