INFLUENCE OF SOLAR LIGHT ON VEGETATION. 
i I 
the closing of flowers and the convolution of leaves is to be attributed to the 
absence of light, and not to the effects of dews; and I regret having mislaid a 
memorandum of the particular flowers which I witnessed partly closed on that 
occasion. I have caused the wild anemone (A. nemorosa) to erect its stem, and 
expand its flowers at night, by placing it between two candles, though the heat 
of warm water had no effect on it ; and a plant of Hibiscus Afrkaniis, that had 
not expanded a flower for several days during the late gloomy summer, though in 
a high moist temperature, gradually opened a blossom when placed near candle- 
light. But my present observations regard principally the effects of light, modified 
by transmission through the glass of hot-houses, and the difference between such 
light and direct solar light. 
The first remark I would make is, that we must maintain a distinction between 
rays of light and rays of heat, for the sun emits rays of these two kinds ; and 
though they both proceed from their source, as it were intimately interwoven 
with each other, yet the effect of transparent substances is to separate them ; for 
one substance may transmit almost every ray of light, and yet exclude nearly 
every ray of heat, and vice versa. 
It is convenient to consider all transparent bodies, — e. g. glass, air, water, and 
ice, — as sieves of infinite fineness, by which the sun's beams are, as it were, sifted ; 
some rays passing through, some being arrested, or absorbed, and others reflected 
back. The atmosphere is an instance of this, as it retains almost all the blue rays, 
letting the others pass through. 
Now, the usual glass of our hot-houses transmits more rays of light than any 
coloured glass would ; but does it also transmit more rays of heat than dark 
coloured glass would ? This deserves consideration; for, if a plant has been sub- 
jected to rays of heat of one particular quality, when exposed to the direct light, 
it will be acted upon by rays of a different quality; and this may cause the 
destruction of its leaves, unless it be gradually inured to the change by shading. 
If any one wishes to prove the difference between the transmission of light 
and heat, let him place a piece of stained black (not superficially blackened) glass 
between the sun and the bulb of a thermometer, and the quicksilver will imme- 
diately indicate the difference between heat thus transmitted, and heat transmitted 
through a piece of common glass. Much more heat, though much less light, will 
be transmitted through the black than through the common glass. 
Again, there are rays issuing from the sun distinct from the rays of heat and 
light : — rays of chemical and magnetic influence ; and who can tell to what extent 
glass may not intercept and transmute these rays ? Hence may arise that inelastic 
state of the air confined in hot-houses ; for no doubt can exist respecting the che- 
mical and magnetic rays of the sun possessing a great influence in regulating the 
component gases of the atmosphere. 
This is not unprofitable theorising. It is a subject lately opened to the scien- 
tific world by the experiments of Messrs. Melloni, De la Roche, and Berard, and 
is of great importance to the horticulturist, who at the present day requires facts 
