SOLAR CHEMISTRY. 
207 
To explain these modern investigations, it will be necessary 
to examine with some care the known conditions of the solar 
agencies, and some of the peculiar phenomena of Light. 
If through a pin-hole made in a window- shutter a sunbeam 
is allowed to pass into a dark room, three phenomena are ren- 
dered evident to our senses. Light, producing vision and 
developing colour, is shown upon every particle of dust floating 
in the luminous pencil. Heat is sensibly felt, if the hand is 
placed in the path of the ray; and Actinism, or chemical power , 
is rendered evident by the change produced, if we allow the 
beam to fall on any photographic preparation. That the sun- 
beam should have the power of breaking up the strongest com- 
binations depending on chemical affinity, is one of the wonder- 
ful discoveries of science. Now, if we place a triangular 
prism of glass in the path of the sunbeam, the rays are bent 
out of their course, or refracted, and by this means decomposed 
into a beautiful flame-like chromatic image. If this solar 
spectrum — this section of a rainbow — is received upon a screen, 
it will be found to consist of several coloured bands ; crimson, 
red, and orange passing into yellow from the least refracted 
end, while from the most refrangible one, we have lavender, 
violet, indigo, blue, and green, also passing into yellow, as 
they advance to the true centre of the spectral image. 
These rays constitute the Newtonian spectrum ; so-called 
from Newton’s having first examined, with precision, the relative 
conditions of these coloured bands, and established, with any 
approach to correctness, the laws regulating the relations of 
colour and refraction. 
Beyond the most refrangible end of this spectrum there exists 
another class of rays, which are not visible under ordinary cir- 
cumstances. If, however, the rays of Light are intercepted by 
solutions of sulphate of quinine, or of horse-chestnut bark, — by 
a block of canary-yellow glass, coloured with the oxide of 
uranium, or by a crystal of fluor spar, those extra spectral 
rays are rendered apparent. Those rays, which were unknown 
to Newton, have been investigated by Professor Stokes, who 
has named them the Fluorescent rays. They are luminous, 
probably under all circumstances, to those animals whose eyes 
are adjusted — as the eyes of most of the night-roaming crea- 
tures are — to admit the rays of highest refrangibility, and to 
vibrate in unison with their vibrations ; but, unless peculiar 
conditions are established, the fluorescent rays are not sensible 
to the human eye. 
Such, then, is the amount of our knowledge respecting the 
luminous principle of the sunbeam. It must not be forgotten 
that the rays of which we have been speaking vary consider- 
ably in the intensity of their illuminating power. The maximum 
