acid; and this fact Professor J.W.
Draper discovered a long time ago by the direct use of the spectrum.
In still further confirmation, we may cite the investigations of
Vogel, Pfeiffer, Selim, and Placentim. The last three have conducted
researches in full knowledge of those of General Pleasonton, and their
experiments show that yellow rays are more promotive of the evolution
of carbon in animals and its absorption in plants than any others in
the spectrum, the violet rays having least power in these respects,
with the exception of the red rays in the case of animals. The
absorption of carbonic acid by plants, and its evolution by animals,
we hardly need add, are prime essentials to the growth and health of
each. The notion that light possesses a magnetizing power on steel was
upset by Niepce de St. Victor in 1861. After removing every source of
error, he "found it impossible to make one sewing needle, solarized
for a very long time under the rays of light concentrated by a strong
lens, attract another suspended by a hair, whether the light was white
or colored by being made to pass through a violet-colored glass."

We can proceed further and even show that violet light is in some
respects hurtful to plants. Cailletet, for example, says in 1868 that
"light which was passed through a solution of iodine in carbonic
disulphide prevents decomposition altogether." Baudrimont says that
"no colored light permits vegetables to go through all the phases of
their evolutions. Violet-colored light is positively injurious to
plants; they absolutely require white light." This scientist
instituted the most elaborate experiments on the subject, ranging over
11 years, from 1850 to 1861; and the result of all his labor may be
summed up in the simple statement that no illumination which human
ingenuity can devise is so well adapted for promoting natural
processes as the pure white light provided by the Creator. So much by
way of general denial of the claims of superior efficacy residing in
blue light of any kind.

Now we have yet to examine the peculiar variety of blue light here
used. Sunlight can, by means of the prism, be split into colored rays,
any one of which we may isolate, and so obtain a certain colored
light. Similarly we may obtain light of a desired color by the use of
a colored glass which will stop out the rays not of the hue required.
So that we may obtain violet light from the spectrum or by filtering
sunlight through