Scientific American, Volume XXXVI., No. 8, February 24, 1877 | Page 9

blue and violet rays are the most active in determining the
decomposition of carbonic acid in plants, and (2) experiments of Dr.
Morichini, repeated by Carpa and Ridolfi, proving that violet rays
magnetized a small needle. The first statement has been totally
disproved. Dr. Von Bezold, in his recent work on color, states that "the
chemical processes in plants, as far as they are dependent upon light,
are principally caused by the rays of medium and of lower
refrangibility. The development of the green color of the chlorophyll,
the decomposition of carbonic acid, as well as the formation of starch,

etc., in the grains of the chlorophyll, are induced by the red, green, and
orange rays." The blue, violet, and ultra violet rays, the same authority
goes onto explain, influence "the rapidity of growth, compel the
so-called zoöspores to move in certain directions, and alter the
positions of leaves, etc." In confirmation of this, we have Sach's
experiments in 1872, which show that light, transmitted through the
yellow solution of potassium chromate, enables green leaves to
decompose over 88 per cent. of carbonic acid; while that passed
through blue ammonia copper oxide decomposes less than 8 per cent.
This proves the superiority of the yellow ray to decompose carbonic
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 violet glass. When, however, Dr. Von Bezold, as above, asserts
that the violet rays have such and such an effect, he means the violet of
the spectrum, which has its specific duty to perform in the compound
light of which it is a necessary portion. But the violet light of the
spectrum and filtered violet sunlight are altogether different things. The
first, as our valued contributor Dr. Van der Weyde has very clearly
pointed out, is "a homogeneous color containing, besides the luminous,
the invisible chemical rays without any caloric rays; while the light
colored by passing through violet glass is a mixture of blue rays with
the red rays at the other end of the spectrum; and it contains a quantity
of the chemical rays belonging to the blue and the caloric rays
belonging to the red. In fact, violet glass passes a light identical with
sunlight, only much reduced in power, containing but a portion of its
caloric, chemical, and luminous agency: being simply deprived of its
strongest