26? 
properties, the three species or portions of light agree, 
yet, in other properties, they very sensibly differ. 
We have already considered the nature and opera- 
tion of the invisible portions of the solar beam, which 
effect chemical changes in bodies ; and we have now 
to direct our attention to that portion which is visible, 
and imparts the sensations of light and colour. In 
the investigation of complicated actions, it is only 
by thus analysing and breaking them down into se- 
parate and distinc tparts, that we can hope to discover 
the full reason and effect oftheir combined operations* 
483. Many examples have been already given of 
the reciprocal action which bodies and the invisible 
portions of light exert on each other ; and the phe- 
nomena of phosphorescence indicate the exertion of 
a similar action between bodies and the luminous or 
visible rays. So, likewise, whether the light afford- 
ed in combustion come from the air, or from the 
combustible body, it equally manifests in bodies the 
existence of a portion of the luminous rays. These 
rays, however, although thus retained in bodies, ap- 
pear to exert on them no chemical action ; for the 
body exhibits no apparent change of properties, 
whatsoever be the colour of the light which it re- 
tainsy or emirs. The emission of light, also, by vege- 
table and animal substances under putrefaction, is 
not the cause, but the consequence, of that process ; 
neither does it appear, that the light afforded by cer- 
tain living animals is necessarily productive of any 
chemical change in the animal, or in the air that sur- 
rounds it ; but merely accompanies those changes, 
which are necessary to the exercise of the vital func- 
