516 
at first, is violet, then blue, and then green, the co- 
lours emerging in the order of the refrangibility of 
the rays ; and as, in this order, the green stands 
before the yellow, it is reasonable to suppose it to 
be, in this case, a simple colour, caused by the trans- 
mission of the green rays, rather than a compound 
of the less refrangible yellow with the blue. Add 
to this, that many of the vegetable colours, as the 
blue, the red and the yellow, must necessarily be 
simple, bee ; : use they cannot be formed out of any 
other colours. But though the green be thus, for 
the most part, considered simple, yet it may, in some 
instances, be compounded, as must be the case with 
the colours of a vast variety of flowers, which pre- 
sent such an endless variety of hues. 
495. As a general summary of the conclusions, 
which, in the foregoing discussions, we have endea- 
voured to establish, respecting the agency of light in 
promoting the colouration of plants, we may ob- 
serve, in the first place, that, by the chemical agen- 
cy (465.) of this subtile matter, the saline compounds 
of plants are decomposed, and the acid and alkaline 
matter, thus developed, combine with the colourable 
juices of the vegetable. In consequence of this com- 
bination, these juices are enabled to act variously on 
the luminous rays. When the alkali predominates, 
the more refrangible rays, as the violet, blue, and 
green, are reflected, and the other rays are extinguish- 
ed; when the acid prevails, the least refrangible, or red 
rays, are reflected, and" the others disappear; and from 
intermediate admixtures of these ingredients, inter- 
jnediate colgurs, both simple and compounded, will a- 
