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

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 zo?spores 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 rays." And this the spectroscope has clearly demonstrated. Reduced to its simplest terms, then, the necessary conclusion is that the violet glass acts purely as a shade for decreasing the intensity of the solar light. And in the simple fact that it does so serve as a shade lies the sole virtue (if any there be) of the glass. In 1856, Dr. Daubeny made experiments on the germination of seeds, and in his report is this suggestive sentence: "In a south aspect, indeed, light which had passed through the ammonia sulphate of