172 
that the height and size of the plant are proportional 
to the intensity of illumination, while its verdure de- 
pends more on the quality of the ray *. 
438. The above experiments, not only prove the 
action of light in the colouration of plants, but de- 
monstrate, likewise, that this action is exerted most 
powerfully by the violet ray. They shew, also, that 
this property of the violet ray is independent of its illu- 
minating power ; but they do not authorise the con- 
clusion, that the height and size of the plant depend 
on the intensity of illumination, for we know that o- 
ther agents contribute to these conditions. By other 
experiments, M. Senebier also ascertained, that the 
heating power of the violet ray had no particular in- 
fluence in effecting these changes of colour ; for, by 
placing thermometers in the differently coloured rays 
of the prismatic spectrum, he found that the violet 
ray possessed less heat than the others, and less, al- 
so, than the entire beam of light ; whence, says he, 
it results, that these properties of the violet ray are 
independent, in a certain degree, both of its heating 
and illuminating power, and appear to depend on 
some particular quality of the matter of which it is 
composed ! 
439. M. Berthollet, likewise, who, as we have seen 
(362.), considers the action of light, in discharging 
colours, to resemble combustion, and thus to cause 
the combination of oxygen with bodies, admits, ne- 
vertheless, many facts which present an apparent 
contradiction to it. It is to the action of the 
Mem. Phys, torn. ii. p. 62. t .Ibid. p. 72. 
