102 
CHEMISTRY OF HORTICULTURE. 
blue ray has passed into, and blended with, the red ray of the lower extremity, and vice 
versa, inasmuch as that red ray is not found equally pure throughout. Again, the orange, 
which is interposed between the red and yellow rays, partakes of the nature of both ; as 
does also the green, which is placed intermediately between the yellow and the pure 
blue rays. 
Philosophers assert that there are but three primitive colours — the red, the yellow, 
and the blue ; and the theory bears out to a certain extent, that of the circular blending 
of those three basal tints. 
Modern researches have partially confirmed the conjectures entertained by writers of a 
bygone period ; and attempts have been made to regulate the introduction of light into 
plant-houses by modifications of the glass. That a powerful influence is exerted by the 
sun's light upon decomposable substances is proved by numberless facts ; colours are 
changed, — those of growing plants particularly so ; attraction is manifested, and leaves are 
drawn up during the greatest power of daylight, as if they were under the attraction of 
an electrical conductor. Of this movement, Erythrina, and many of the trifoliate Legu- 
■minosa;, may be cited as most familiar examples. Heat also is developed, though the 
transparent medium itself, through which the rays pass, is not sensibly affected, unless it 
be by radiation from a surface below, upon which the sunbeams have struck. In a word, 
we may safely conclude that the sun's rays invariably act as a divellent decomposing 
power. 
We have now arrived at a position whence we can obtain a glance of the chemical 
and electro-magnetic principle of the sun's light. It has long been suspected that 
the blue rays produce magnetic phenomena, and needles have been said to be thereby 
converted to magnets. The blue ray, at all events, developes little heat, and it may not be 
incorrect to suppose that cold frosty weather may depend upon its predominant influence. 
The illuminating yellow or central rays may be the more immediate source of electricity; 
as the red was formerly supposed to be the agent of heat. But more correct experiments 
have proved that the greatest heating power is manifested a little below, and out of the 
visible range of, the red ray. In a word, keeping in view the entire range of the prismatic 
spectrum, it will be found, that, if a delicate thermometer be applied to the different 
coloured spaces, the heating power benig at its maximum beyond the limits of the red ray, 
decreases gradually through the various tints till it arrives at the blue spaces, where it is at 
the minimum. 
Chemical changes, chiefly attended with decomposition, are produced by the action of 
light : one only need be adduced, as familiar in domestic economy. When the common 
permanent ink (which consists of metallic silver dissolved in dilute aqua-fortis, coloured 
with a little Indian ink) is applied to linen, it is of pale brown colour, and could imme- 
diately be discharged ; but after exposure to the full sun's ray it acquires a fixed black tint. 
In this process the solution becomes decomposed by the agency, chiefly, of the blue ray, 
and the silver is partially restored to its metalhc condition. Now, according to Dr. Fownes, 
"it is not the luminous part of the ray which effects similar changes ; they are produced by 
certain invisible rays accompanying the others, and which are found most abundantly in 
and beyond the violet part of the spectrum. It is there that the chemical effects are 
most marked, although the intensity of the light is exceedingly feeble. The chemical 
rays are thus opposed to the heating rays in their degree of refrangibihty, since they exceed 
all the others in this respect." 
According to the most recent discoveries, it should appear that white, undisturbed 
light, consists of at least three active constituent principles — namely those of light or 
luminosity, heat, and actinism. We may, perhaps, never be able to separate or define 
these principles ; and as regards vegetable growth and maturity, it is not essential that we 
should do so, because softened white light imparts every element which can be required. 
I say softened, because of the evil consequences that result from the burning lenses that 
are found in common or sheet glass ; and I use the term with a view to persuade the 
horticulturist to substitute ground glass for any screen, be the fabric what it may ; because 
light is, and must be, intercepted by the latter, and its qualities more or less changed. 
