217 
rise. The colours, however, which these juices pre- 
sent to our sight, are not reflected by the coloured par- 
ticles (417.), but by the opaque matter on which they 
are imposed, so that the coloured matter transmits 
only, but does not reflect light ; and this light arriv.. 
ing at the eye, produces an impression, which con- 
veys the sensation of the individual colour.* 
496. Hence, too, it follows, that, when light is 
wholly excluded, the chemical changes in the vege- 
table juices, which enable them to exert these actions 
on the colorific rays, do not take place, and, conse- 
quently, the green colour of the leaves, which de- 
pends on the predominance of alkaline matter, and 
the red colours of leaves and of flowers, which arise 
from an excess of acid, are equally prevented from 
appearing ; for the juices being unable, in this state, 
to decompose the solar beam, return it almost or en- 
tirely unchanged to the eye, whence the objects are 
destitute of colour, or have the appearance of white- 
ness. The colours of plants, therefore, depend pri- 
marily on the chemical action of light, in changing 
the constitution of their juices ; and these juices, by 
their physical operation on the colorific rays, are 
then enabled to exhibit all their infinite variety of 
hues, 
497. We are fearful that some of our readers may 
think we have extended these remarks on the agency 
of light too far, and that many of the observations, 
which have now been made, are but remotely connect- 
ed with the professed objects of our inquiries. We 
entreat them, however, to suspend their judgment, 
