Light

LIGHT. Light is that natural force which, acting on the retina of the eye, produces the sen sation of vision. Upon light, the continued vitality and growth of plants is dependent, and in its absence animals soon lose vigor, and at length their health is broken. In agriculture, the growth of plants is dependent upon the ,quantity of light received, and hence in seasons when the sun is much obscured by clouds, sensibly suffers. In the effort to produce seed, although if sufficient heat be present, the vegetation may be luxuriant, but will lack in vigor. The sun is the great source of light and heat, for so far as vegetation is con cerned, both are necessary to successful growth. Besides the sun, all the planetary and starry sys tems give off light, but not appreciable heat, .although some late experiments with delicate instruments leould seem to show that appreciable heat was thrown back by the moon. Plants -during light nights have been observed to grow faster than during dark nights, hut experiments in this direction have not yet been carried forward .sufficiently far to prove this beyond doubt. The principal phenomena and discoveries in the relation of light to vegetation, have been com piled by Prof. Poey, which will be interesting. The condensation of the principal experiments and facts are given below as follows: The paleness or whiteness of the parts of vege: tables, when they are sheltered from the action of light, called by the moderns the etiolation of plants, did not escape the observing mind of Aristotle, who described it correctly enough, • only he wrongly believed that the roots owed their paleness to their underground position; for they do..not become green when exposed to light, and those which grow there are white. Still some roots are in themselves green, with or without the production of green matter or chlorophyl. The celebrated botanist, John Ray, first among the moderns, proved in 1686, by many experiments, that light alone exerts an influence upon the green color of plants, that they scarcely vegetate at all under an opaque vase, and that their stems become extremely elongated; this phenomenon was neither owing to the want of air, nor to the influence of heat. Charles Bonnet established that it was owing to the dark. Meese and Senebier analyzed the cause and the circumstances. Michellotti also first proved that light was injurious to germination, and Ingenhousz and Senebier found that seeds _germinate more rapidly in- the shade than in sunshine. After the tendency of stems to turn toward the light, other phenomena were discovered at the commencement of this century relating to the influence of solar rays upon vege tation. Thomas Andrew Knight indicated for the first time in 1812 the tendency of some stems to shun the light in place of seeking it, and _Dutrochet demonstrated it in the most incon testable manner. Dutrochet himself pointed out, in 1824, the tendency of roots to turn toward the light, and, in 1844, J. Payer dis covered the contrary disposition in other roots to shun the light. After recognizing the action of natural and artificial light upon the growth of plants and the production of green matter, it will be interesting to study the influence of the different colors of the spectrum upon vegeta tion, by the aid of colored glass and liquids, and by the colored rays of the solar spectrum. Sene bier, who appears to have been the first to take up important mportant question, published in 1783 experi ments which he began seven years before, which were continued during four years. His conclu sions are as follows: 1. The size of the stems and their whiteness are so much the greater as the plant's illumination is less; but the green color of the leaves does not follow this rule absolutely. 2. The highest illuminating rays, the yellow and red, do not give the leaves the most decided green tint; the violet ray produces this effect with the lowest illuminating power. Senebier draws the conclusion that the green color will be deeper , when there is less light reflecting from the leaf and more penetrating it, and on account of this the violet ray gives the leaves a deeper green than natural light. 3. After the violet rays, plants'become more de veloped in the red rays4han in the yellow. By submitting plants to the action of light trans mitted through glass of different colors, l'Abb6 Tessier remarked, in 1781, that the intensity of the green tint of leaves went on decreasing from white glass to deep yellow; that leaves become sensibly green under the influence of lamps and of the light of the moon. He also observed that the stems are very strongly inclined when we place black materials behind the plant; the inclination is very feeble when the stuff is white; and it is almost nothing when we place a look ing-glass behind the vegetable. Leaves which were placed hehind a pane of white glass were green; less, however, than if the pot had been outside of the apartment. Leaves behind a pane of blue glass were still more green, there being between them and the preceding a sensible shade. Leaves behind a clear yellow pane differed from the last only by being a shade less green. In fine, the least green of all were those which had been placed behind a deep yellow pane; the difference was striking. In 1817, Dr. Sebastian Poggioli, instead of submitting plants to light transmitted through colored glass, conceived the happy idea of placing them under the rays of the solar spectrum. He observed, after the third day, that the smallest plant submitted to the violet rays, surpassed, in development, those under the influence of the red rays, the latter, on the fourth day, appearing to he blanched. The cotyledonous leaves of the plantlets in the two vases changed their positions and directed their upper surfaces one toward the red light, and the other toward the violet. The direction was more rapid with the plantlets submitted to violet rays than with those which had been submitted to red rays. The two cotyledonous leaves which crown the plantlets of Rapltanus rwticanus directed their upper faces, in the first place, toward violet light, and in the second, toward red. The seeds of Brassica eruca ger minate more promptly under the influence of red rays than under green rays, and more promptly under green than violet rays. In 1830, Adolphe Brogniart and de Jussieu saw the two opposite leaves which terminate the plantlets of Alsine media direct in the same manner their upper faces toward the red light to which alone they were submitted. Thus the phenomenon was the same as that observed twenty-seven years before by Poggioli. Brogniart remarks that he had observed, as had Charles 'Marren, that the yellow rays approached, in their action, nearer than any of the others to white light; but, as this depended upon the fact that yellow glass allowed a very large quantity of white light to pass through it, while green and blue passed but a little, and red glass none at all, the intensity of light differed extremely in all these experiments. In 1832, Charles Morren, by the aid of experiments with colored glass, reached the following conclusions: 1. That the colors of the spectrum, excepting green, only, like darkness, assist germination as their luminating power becomes less. 2. That under the colored rays of greatest illuminating power, the radicles develop least and slowest, and that the plumules, on the contrary, grow best and quickest. 3. That under the rays with low illuminating pow ers, the radicles and plumules develop as they do in the shade. 4. That, consequently, the etiola tion of vegetable% under the prismatic rays, happens in inverse proportion to their illuminat ing property. 5. That the green color of vege tables is developed much more rapidly under the influence of composite light than under depom Vosed rays. 6. That the yellow rays possess the maximum, and the orange rays the minimum degree of coloring plants green, and the other rays do not turn them green at all. 7. That the yellow ray greens them more when it is less intense, but that it requires much more time to pro duce this effect than white light does, and that it can never produce the same depth of color. In 1837 and the following years, Dutrochet made a great number of experiments upon the influence of light on vegetation. None of the stalks of Lepidium, sativum showed the least inflection toward light transmitted by red glass., At the end of eight days they had increased consider ably in length, but remained straight and verti cal. These observations completely confirm those of Payer upon the same plant. Qn the contrary, the stalks of Alsine media were, at the end of three dui, all turned toward light trans mitted through red glass. On turning the vase in such a way as to direct the inflection of the stalklets toward the base of the apparatus, four hours afterward they were bent anew toward the red light. Dutrochet then proved that this depended on the size of the stalklets of the two plants; the only plants bent toward red light were those whose diameter was less than fifty-five hundredths of a millimetre; a tenth of a millimetre difference between the diameter of one stalklet and another of the same species, near a certain limit, sufficed to bend one toward the light, and leave the other in its natural posi tion. The experiments of Dutrochet prove that the stalks of plants are bent toward red light transmitted without mixture by glass of this color; while, according to Payer, they inclined toward blue or violet light. Dutrochet here raises an important question, whether it is not the luminous intensity of the colored rays which determines the inflexion of vegetable stalks, an intensity varying with the transparency of the glass. The first flexion in the stalks toward the light is shown without exception in those suh mitted to the violet rays; afterward, in the indigo and blue rays, and ordinarily at the same time in the lavender rays which form the almost invisible continuation of the spectrum beyond the violet; then in the yellow and green rays, in the orange, and finally in the red rays. Beyond violet the flexion of stalks reaches twenty and sometimes even thirty centimeters, according to the intensity of the light; but beyond the red rays the flexion of the stalk extends very little. Previous to 1837, Dr. John William Draper, of New York, having made several experiments on the action of different rays of the solar spec trum, concluded that the yellow ray, the most luminous, had the power of producing chlorophyL Plants become also green in light that has been 'submitted to the action of the yellow solution of bichromate of potassa, and so deprived of those rays which blacken chloride of silver; the same is true of light which passed through sulpho cyanate of iron, and sulphate of copper and ammonia; in every instance the leaves becoming green. Seeds of common cress were caused to germinate and grow under these circumstances. It is not this or that ray, adds Draper, which gives rise to the color of beans. The absence of the chemical or the calorific ray does not appear to affect it, nor have we any direct proof that the calorific ray exerts any influence. Humboldt has stated that in the mines of Germany plants grow in recesses where the sun's light never comes, and, provided hydrogen gas be present, the color is green. In the abysses of the ocean, at depths to which no solar beam can penetrate, and where there is perpetual night, green plants are found flourishing. Light, which seems to act merely as a stimulus on the green organs of vegetables, indirectly bringing about the decom position of carbonic acid, though accessory-, is not, however, essential to the growth of plants. From 1840 to 1847 Robert Hunt was engaged in an extensive series of experiments on the process of germination and vegetable growth as affected by solar radiations. These experiments were undertaken under the auspices and at the expense of the British Association for the Advancement of Science. The results to which he was led are: 1. Light prevents the germination of seeds, as was formerly asserted by Michellotti, Ingen housz, Meese, Senebier, and others. 2. Actin ism, or chemical rays, quickens germination. 3. Light acts so as to effect the decomposition of carbonic acid by the growing plant. 4. Chemi cal action and light are essential to the forma tion of the green matter of leaves, or chlorophyl. 5. Light and chemical rays, independently of the calorific rays, prevent the development bf the reproductive organs of plants. 6. The heat radiations corresponding to the extreme red rays of the spectrum facilitate the flowering of plants and the perfecting of their reproductive prin ciples. As to the conditions in which the lumi nous, chemical, and caloric principles exist in light during the different seasons, he has found that—in the spring, chemical action is the most powerful, and is in very considerable excess of pure light and heat. As summer advances the quantity of light and heat increases to a very great degree above chemical action. In the autumn, light and chemical action both diminish, and the calorific radiations predominate. These facts bear strikingly upon all Hunt's experi ments; they appear to confirm his conclusions in a most satisfactory manner, and also to point to a singularly interesting order in nature, which is the following: In the spring, when seeds ger minate, and young vegetation awakes from the repose of winter, we find ati excess of the prin- ' eiple which imparts the required stimulus—that ig of chemical action; seeds then germinate, and ' young buds and shoots are developed. In the summer this agent is counteracted by another possessing different powers, upon the exercise of which the formation of the plant's structure depends—that is luminous action. As the sun becomes more active the formation of woody, fiber proceeds under this agency, and the chemi cal power is rendered less active by the purity of the light. Finally, in the late summer and in the autumn these are checked by another agency with diminished power of light, upon which the development of the flower appears to depend, the ripening of the fruit, and the perfection of the seed --and this is thermic action. The experiments of Hunt have also led him to detect some curious influences which appear due to dissimilar rays, and in their action exhibit great inconstancy of effect. One class of rays, the same to which Sir J. Herschel has given the name of parather mic rays, are so subdued by the influence of ,more refrangible rays, as to be nearly inactive during the spring and early summer months; indeed in the spring they scarcely produce any effect upon dead vegetable coloring matter, unless their action is assisted by the ustof some decomposing agent such as sulphuric acid. These rays increase in power toward autumn, and to them appears due the browning of the leaf. In all his writings Hunt has repeatedly stated that seeds would not germinate under the influence of light, when deprived of that principle on which chemical Change depends; and has declared as a law, that in its relation to vegetable life, light preventsgermination. This statement has been decidedly objected to by Doctors Draper and Gardner, of New York. By numerous experiments Hunt also found that the chemical principle of solar radiation pro duces an acceleration of the germinating pro cess, so that in everyinstance the seeds influ enced by these chemical rays germinated in one half the time required by the seeds placed in the dark. In ascertaining if the influence of the chemical rays was confined to the surface of the soil, or extended below it, he obtained the most satisfactory evidence, that under the rays which passed through blue glass, germination began at a depth below the surface, where it did not take place under' ordinary conditions. In every instance germination through the agency of radiations, which had permeated the blue glass in less time, and to a greater depth in the soil than was possible in comparative experiments where the seed was exposed tq the full influence of light and its associated radiations as combined in the solar beam. As to different colors of light, Hunt says, that if the young plant continues to grow under the influence of blue rays, it will for some time exhibit luxuriance and present in its earlier stages an appearance far superior to that of plants grown under other influences, or even white light. The leaves will be of a darker green, and altogether show external signs of vig orous health. Even in•the earliest stages it will be found that the plants grown in the full sun shine, or under yellow or red representing the luminous and calorific principles, give a larger quantity of woody fiber and less water than those grown under chemical influence. Plants growing in the shade, in like manner, contain more water than those in full sunshine • ce hen, we can not infer that anything more than the deprivation of light influences the condition of plants in these experiments. He ascertained that as great a difference exists in the quantity of water found in plants growing in artificial shade, and plants of the same class growing unshel tered, under the ordinary conditions of sunshine. It is iherefore evident that all those experiments, which have been made on the increase of grass crops by littering the fields with the boughs of trees and the like, are liable to the error alluded to; and it is questionable if the increased pro duct of an acre is not principally due to water, rather than to any carbonaceous product; conse quently the increase of the nutritive property is not in the ratio of the increase of weight. Hunt further observes that it frequently happens, when the media employed cut off a large quantity of light, and admit the chemical principle freely, that no formation of leaves takes place after the development of the plumule or first leaf bud; the ground stem, instead of solidifying, remains soft, and without increasing in diameter, extends to an enormous length. Nothing like this occurs under the influence of either light or heat. It would appear that this abnormal condition is due to the excitement of the chemical rays, acting possibly with great power upon the living principle in the roots, by 'which the elaboration of some organizable matter is pro duced, which they supply as food to the stem. As there is but little power to decompose carbonic acid, there is not the necessary supply of carbon to give rise to those steins and leaves which naturally form the primitive stem. That this is something like a true explanation is fur ther proved by the fact, that in the practice of planting shoots, the use of blue media is bighly advantageous. It appears to increase the ten deney to develop roots, and it is satisfactory to learn that some gardeners have, without any knowledge of the cause, employed cobalt blue glass to aid in the striking of cuttings. Accord ing to Senehier, plants decompose the carbonic acid absorbed by their leaves much more readily under the influence of violet rays than under any other. In his early experiments Hunt thought he had found the correctness of Senebier's opinion. Dr. Daubeny and Dr. Gardner both think that the decomposition of carbonic acid increases with the increase of light, and that it is more rapid under the influence of the yellow ray than any other. In the experiments of Hunt, by which the quantity of woody fiber increases with light, this agent seems to be essential to its formation. But Hunt explains this fact by say ing that the decomposition of carbonic acid by plants under the agency of light is simple chemical' operation, as has been supposed by some, effected by the, chlorophyl, but the result of an exertion of the vital principle of the grow ing plant whiCh requires the external stimulus of light to call it into action. Hunt seems to have discovered a capital point, to which he has not hiniself done justice, and Which has passed entirely unacknowledged by other observers. It is that in all his experiments he has found that each variety of plants employed was influ enced by different rays. Cress and mustard, for example, become green most rapidly in the green ray, mignonette in the yellow, and peas in the blue ray. He observes, however, that the influ ence was always most decided between the limits of the mean orange and the mean blue ray, and that it took it much longer to green plants in the red than in the blue ray. The importance of this discovery, should it be con-, firmed, will consist in showing the action of light upon plants to be identical with that of heat. If each plant, as we have seen above, possesses its own zero of temperature and requires a certain different degree in order to accom plish each of its physiological functions, in light the greening of the leaves or production of chlorophyl (and probably the decomposition of carbonic acid and the fixation of carbon) needs also the chemical action of a certain colored ray of the solar spectrum. The difference existing between each color of the spectrum being only a difference in greater or less velocity of vibra tion, and of properties purely relative, as in heat and light, each plant will be therefore chemi cally affected by rays of different colors. Hunt thinks that the manner in which the power of solar radiation is exerted on seed beneath the soil is not clear to us. We do not know whether it is a mere disturbance of something already diffused through matter, or in the seed, or an emanation from the sun. All which we are ena bled to declare is that the germination of seed is more rapid under the influences of chemical rays, separated from the luminous, than it is under the influence of the combined radiations or in the dark. That the formation of chloro phyl is not directly, dependent upon either light or chemical action, regarded as isolated princi ples, but on the action of both forces acting upon plants. It has been stated by Dr. Gard ner, adds Hunt, that plants exhibit a lateral movement bending toward the yellow ray. This appears to be a mistake; plants under the influ ence of the red rays bend from the light but along the line of the ray; and those exposed to the most refrangible rays turn toward it, but still in the line of the ray. Now the plants which

first become green, by careful treatment in this way, are those exposed to the rays between the mean Freen ray and the extreme blue. The action is continued eventually to the edge of the most refrangible violet below the yellow ray. No change is effected beyond the visible spec trum, notwithstanding the abundance of dark chemical rays; and the change is shown only where there is really the largest amount of light. He therefore concludes that the lumi nous rays are essential in producing the decom position of the carbonic acid and the deposition of carbon, which is afterward, in all probability, continued with hydrogen under the influence of the purely chemical rays of solar light. Finally, Robert Hunt communicated a letter to the Brit tish Association at its meeting in 1853, from Me. srs. Laws and Co., of Edinburgh, in which it was stated that by adopting the plan of cutting off luminous rays by the use of cobalt blue glass, as recommended by Hunt, they succeeded in obtaining healthy germination far more rapidly than in ordinary circumstances. They con structed a house glazed with blue glass, and in this their seeds were tested. The practical application of a scientific discovery was of the utmost value to them. Tropical seeds, under the same circumstances, were found to germi nate in a few days, when in ordinary conditions many weeks were required for the process. According to l'Abb6 Zantedeschi, Carradori had, in 1841, confirmed the results of Senebier, so fir as that the power of light to color vegetables green, resides in an eminent degree in the violet and blue rays, although it is more feeble than in ordinary light. In 1843, J. Payer, by making use of colored glass, and afterward of the solar spectrum, found that the tendency of stems toward the light was so much greater as the light was less intense or reached a lower point. AS the plant found between two luminous rays of dif ferent intensities is always curved on the side of the stronger light, between the blue and violet, the blue region has always the greater amount of this action, and in it is found the maximum. But Payer has not studied the action of invisi ble chemical rays beyond the violet, upon the flexion of the stems, as Guillemin has done later, although Hunt found no influence upon vegetation in the ultra-violet rays. Hence, Prof. Poey is also inclined further to believe that Hunt has not paid attention enough to the obscure and chemical rays. In 1842, l'Abb6 Zantedeschi drew the following conclu sions from his experiments upon the influence of solar rays transmitted by colored glass: 1. Vegetation under the influence of light transmit ted by any kind of colored glass is languishing and sickly, as was observed by Senebier and Car radori. 2. The order observed for germination, under colored glass, is different from that observed by Senebier. 3. Violet light has a power little inferior to that of ordinary light to green certain vegetables, as Senebier has said: the balsam is in this condition, but it does not take place with Oxalis multiliora. 4. As to the vigor of vegetation it is no greater under violet glass than under yellow and red glass, as Sene bier has observed. 5. Green light is less favor able to vegetation than red light. 6. The most vigorous vegetation for Oxalis multillora takes place under blue glass. In 1844, Dr. D. P. Gardner, of New York, claimed to have settled the question beyond dispute, that the yellow rays produce chlorophyl in leaves, as Dr. Dra per had first advanced; but he is in complete disagreement with Hunt and other experimenters. Here are his conclusions: 1. That chlorophyl is produced by the more luminous rays, the maximum being in the yellow. 2. This forma tion is due to pure light. 3. That the ray toward which plants bend occupies the indigo space in Fraunhofer's lines. 4. This movement is due to pure light as distinguished from heat and chemical action. 5. That pure light is capable of producing changes which result in the development of palpable motion. 6. The bleaching of chlorophyl is most active in those parts of the spectrum which possess no influ ence in its production and are complementary to the yellow rays. 7. This action is also due to pure light. In the same year, Robert Harkness, of England, objected to Gardner's conclusions, and made the following very judicious remarks, which were approvingly quoted by Hunt: If the absence of solar light is one of the condi Lions almost necessary for the germination of seed, one should not expect that ray in which the maximum of light is found, to 'facilitate germination; but on the contrary, according to Hunt, to retard it. If, again, the yellow ray is the operating cause by -which carbonic acid is decomposed and chlorophyl produced, we should also expect that, so far from assisting in germi nation, it would exercise a highly injurious influ ence. We know that the presence of oxygen is necessary for the vegetation of seeds, also that this oxygen is absorbed, and by uniting with a portion of the carbon in the seeds reappears in the form of carbonic acid, a process the oppo site of that which takes place when chlorophyl is produced. So if the decomposition of carbonic acid is owing to the yellow ray, this ray ought to be the last to produce any effect on the germina tion of seeds. Moreover, seeds, as well as fully developed vegetables, possess the power when deprived of light, of absorbing oxygen and evolv ing carbonic acid; and to this circumstance we must ascribe the effect of the blue ray shown by Hnnt's experiments. In 1850, Cloez and Gratio let advanced the opinion that the decomposition of carbonic acid by the green parts of plants with the aid of colored glass is at its maximum with colorless polished glass ; next comes yellow glass, then colorless transparent glass, red, green, and in the last place blue glass. In 1852, J. H. Gladstone, in experimenting with peas, drew the following conclusions, in entire opposition to Hunt's experiments: The cutting off of the chemi cal 'rays favors the first germination of the seed, and this appears to be the principal, if not the only advantage obtained by burying the seed in the soil. The development of roots also requires the absence of the chemical ray, yet it does not go on to the greatest extent when all the solar influences are excluded, but is favored, rather than other wise, by heat and luminosity. The first devel opment of the plumule also proceeds best under the same circumstances; yet these are not the con ditions which produce a healthy plant ; If all the solar radiations are withdrawn, whether en tirely or only to a great extent, the plants absorb much water and grow very tall, without devel oping secondary branches or many leaves. The whole force of these radiations, on the contrary, prevents or greatly impedes the growth of those plants under the circumstances of the experi ments. The chemical force is the most antago nistic to, the growth of the pea, and luminosity also militates against it; the heating rays are fa vorable. If the plant is fairly established, those radiations which are, comparatively speaking, devoid of light, but replete with chemical power, are the most suitable to a healthy growth. The influences which facilitate rapid growth are dia metrically opposed to healthy development. Upon the question whether the yellow light stops germination by some specific action or merely by excess of light, Gladstone says that the yellow light did not at all interfere with germination in his experiments. In the case of wheat and peas it decidedly facilitated the early development of ,both root and plumule. He thinks that the yellow ray, however, has a specific action of its own, and that the yellow and obscured yellow give quite different results from those of any other glasses. On the whole. Gladstone's experi ments are in accordance with Draper's. In one or two cases, the seeds exhibited a tendency to ger minate more readily under a blue glass. In 1854, Robert WaShington, in some experiments made ou the influence of colored glass on the growth of plants in sea-water, found that red sea-plants grew' est in cases whose light was allowed to pass through green colored glass, and that the brown and green confervoid growths were thus de stroyed. The first observer, as far as I am aware, who studied the action of the ultra-violet or in visible rays of the spectrum upon vegetation, and according to exact scientific methods, was C. M. Guillemin. His first care was to vary the nature of the prisms which he employed, in order to turn to account the special transparency that each prismatic substance presented to rays of differ ent refrangibility. He chose, for instance, quartz for all rays more refrangible than blue, rock-salt for the red and calorific rays, and flint for the mean rays of the colored spectrum. Heavy flint which he had also employed, is the most disper sive of all; but it absorbs in great part the calo rifle and ultra-violet rays, transmitting only the least refrangible of them. In a paper presented to the Academy of Sciences of Paris, in 1857, Guillemin reached the following conclusions: The ultra-violet rays determine the formation of the green matter of vegetables. These same rays effect the flexion of the stems more rapidly than the rays of the visible part of the spectrUm. In a second paper to the same academy he acknowl edged that in his first communication he had given as certain the first proposition, but that he had some doubts on the second ; further re searches, however, had fully confirmed both of these prior propositions. His conclusions arc the same as those he afterward published in a third, more extended work. They areas follows: Young etiolated plants curve under the influence of all the rays of the solar spectrum; the least refrangible calorific rays or those of low temperature appear to forth the only exception. The calorific rays less refrangible than the red, and the chemical rays more refrangible than the violet, present two maxima of action for the flexure of vegetable stems; intermediate colored rays determine, on the contrary, more actively than the preceding the formation of chlorophyl. In the spectrum obtained with a quartz prism, the limit at which the flexion of the stem ceases, passes beyond that of rays more refrangible than violet, indicated by fluorescent substances and iodide of silver. Lat eral flexion extends beyond the red and violet extremes ; its center is in the indigo rays ; it is often produced in spite of screens which separate the different colored rays. The development of green matter has its maximum in the yellow; it diminishes slowly as it proceeds toward the vio let, passes this limit and becomes nil in the last fluorescent rays. Blue, green, yellow, orange, and red rays turn etiolated leaves green more quickly than direct solar rays. The action of yellow is almost equal to that of diffused atmos pheric light. Polarized rays appear to act at close intensities, like common rays. The principle of the identity of radiations, which already rested on the observation of a large number of physi cal phenomena, is here fully confirmed in the physiological order, by the analogy of the mode of action which the Calorific and ultra-violet rays exert upon the flexion of stems and the develop ment of green matter. Unhappily for these important researches, if we compare Guillemin's first conclusion with the eighth of his second note, we find a flagrant contradiction upon a question of the highest importance, the maxi mum action of the colored ray of the solar spec trum, under the influence of which the formation of the green matter or chlorophyl takes place. According to the first conclusion it is in the ultra-violet rays that this is produced, while in conclusion eight it is in the yellow ray, and even diminishes slowly proceeding toward the violet. Thus it is wholly the reverse of the first case. Such a contradiction throws a weighty doubt on the whole of these experiments. In the follow ing paragraph we see that Prof. Henry agres with Robert Hunt and Prof. Poey as to the nox ious influence of light on the germination of seed: Germination can be carried on in the dark, and, indeed, the chemical ray which accompanies light retards rather than accelerates the process. Its office is to separate the atoms of carbon from those of oxygen in the decom position of carbonic acid, while the function of the power within the plant results from the com bination of these same elements. The forces are therefore antagonistic, and hence germina tion is more rapid when light is excluded; an inference borne out by actual experiment. In 1869; Dell6rain proves that the evaporation of water by the leaves of plants takes place under conditions entirely different from those which regulate thb evaporation of an inert body, as it occurs in a saturated atmosphere; that it is espe cially effected by light; and that the luminous rays efficacious in causing the decomposition of car bonic acid by the leaves, are also those which favor evaporation. The yellow and red rays, which have little action on photographic paper act with most intensity in causing the reduction of carbonic acid, while the blue and green rays decompose the chloride of silver, and have no action on the leaves. These experiments confirm the old observation of Guettard, that the hard and smooth upper part of the leaves evaporates the most water. Boussingault has shown that the greatest amount of carbonic acid is decom posed by the same portion. Prof. Poey con cludes as follows: After the long series of experiments I have announced, and which each one should utilize for himself, I must formulate my own conclusion, which I advance only with the title of hypothesis until I have the means of submitting it -to experimental test. I remark first, that these experiments have been performed after different methods, and are therefore not to be compared with each other. Some 'experi menters have made use of colored glasses, others of colored liquids or Solutions, and others still of the solar spectrum. In these three, methods the nature and action of the colored rays must have undergone modifications so various as to make the results very discordant. For example, the most of colored glasses are impure, because they allow white light and Various tinted rays to pass besides those of their own proper color. Yel low and green glasses, which are usually too clear, allow a great amount of white light to pass. The thickness of glass, the density of liquids, and the composition of analyzing prisms, are points of the highest importance, which no one has taken into account, and which modify greatly the tints of colored rays. Another cause of error is, that each observer is contented with submitting the plant to the action of this or that colored ray, and not to each of the seven colors of the spectrum. In short, no observer except. Guillemi?, not even Hunt, conceived the idea of studying the influence of invisible rays beyond red and violet, which afford i the first, the maximum of calorific action•, the second, the maximum of chemical action. These are errors of method only, without counting the personal errors of experiment, or even the errors which could result. from the physiological and physical action of the plant. We have already seen in the case of Dutrochet, that one-tenth of a millimetre's dif ference in the diameter of two twigs of the same species was enough to bend one toward the light and the other away from it. In analyzing the action of light on vegetables, three great physio logi&,1 acts of the plant must be considered: 1 germination of the seed.. 2. The decomposition of carbonic acid from the atmos phere and the fixation of carbon by nutrition in the tissues of the plant, which results in the for mation of chlorophyl. 3. The flowering and fructification. In germination, according to. Theodore de Saussure, we should 'distinguish two. periods, the one during which oxygen is trans formed into carbonic acid, which is the period of germination, properly speaking; the other during which• carbonic acid is decomposed and oxygen is exhaled, which is the commencement of vege tation. The green color in which the plant clothes itself is the index and the consequence of that function which takes place only under the action of light; in its absence the plant continues the phenomena of the first period of germina tion without development; aud remains color less. If the physicists who have studied the influence of light on germination had taken the chemico-physiological action which it wrought, in the plant more into consideration, they would not have committed another grave error, as. Gladstone has done, in saying that the yellow rays, which possess the maximum of luminous. power, and by consequence the minimum of chemical force, could in any manner favor the ger mination of seeds. The physicists who sustain this opinion have not sufficiently distinguished germination from vegetation, properly speaking. During germination the plant loses the carbon furnished by the carbonic acid, and only respires. On the contrary, during vegetation the plant retains the carbonic acid, fixing the carbon and drawing in oxygen, and therefore the functions. of nutrition and assimilation of plants do not really begin till the moment of the appearance of green matter. The relations of plants and light are altogether different during the two periods; injurious during the period of germination, light is, on the contrary, indispensable in the second period of vegetation. It is for this the plant seeks always after light. Its shoots cling toward the side of incidental rays and inflect themselves constantly in that direction. It is a. phenomenon general in plants, from the geniis to the cryptogamia, and in microscopic vegetation. Light is as indispensable to the nutrition of plants as the 4 i .astric juice to digestion and the transformation of food. The assimilation and fixation of carbon in plants is greatest and most rapid when, all other thine being equal, the supply of light is greatest. In the light of the solar spectrum, besides the special property of the seven colors which reduce themselves to three primary ones, three feces also are found intimately 0,ombined and worktag simultaneously. These are the luminous, the calorific. and the chemical force. Each one of these thlee great powers acting diversely on inorganic matter, there is no reason why they should not act equally after their nature on organic matter, whose chemical elements are the same. In fact, modern chemistry conclusively demonstrates that the vegetable, like the animal kingdom, is formed of but four elements, carbon, oxygen, hydrogen, and nitrogen. These four elements being combined in the most diverse and variable relations, it results that an innumerable quantity of organic components enjoy properties most distinct. On the other side, these four simple bodies are precisely those which consti tute the atmospheric air. We can say with i Dumas, that the whole vegetable kingdom is only the air condensed, and that it forms a vast apparatus of reduction. The base of these vegetable productions is always the same, the atmospheric air; the arrangement of molecules and their number is all which varies for consti tuting that mysterious force which one com monly calls life. From these considerations we establish hypothetically-1. That it is the purely chemical rays or ultra-violet with the aid of a Certain quantity of obscure heat found mixed with the rays, which penetrate the vegetable stratum of the soil and determine the germina tion of seeds. 2. It is the purely luminous rays of the spectrum, the yellow, which determine, principally by the leaves, the decomposition of carbonic acid from the atmosphere and the roots, the fixation 9f carbon and the production of chlorophyl, and which explains the spontaneous tendency of the plant to the light. 3. It is the purely, calorific or ultra-red rays which determine the two physiological acts of flowering and fructification. Now the blue and violet rays on one part,' the green on another, and the red rays have a purely secondary action in the production of these three great physiolog ical acts of the plant, as the chemical, luminous, or calorific actions predominate in the second place. But it must be remembered before all that what we call the vital force proper to the plant precedes and modifies at will the three actions of the solar rays, chemical, luminous, or calorific in all the physiological acts of the vegetable kingdom. , If this is true, we see at once the application to be made to agriculture and horticulture. First,' in the arrangement of greenhouses, which leaves much to be desired at present. They should have compartments covered with pure glass, whose maximum chem ical, luminous, and calorific properties would be more advantageous to each development of the plant. For example, in germination the glass should have chemical properties of the highest degree. For coloring the leaves and giving growth to the plant, glass should have the maximum of light; for flowering and fructifica tion, the maximum of heat. As for the colors of the spectrum, the blue-violet, the yellow, and the red will yield the most advantage in these physiological acts of the plant. In general practice, however, these conclusions of Prof. Poey have not been found applicable, and yet in certain cases, especially in the case of plants requiring peculiar conditions, they are worthy of consideration. A few years ago prominent individuals, apparently jumping to conclusions, advocated the adoption of blue glass indiscrimi nately and as a means of securing health to invalids as well as plants. It resulted in a great sale of blue glass, but the great results not com ing about that was expected, the sale soon ceased. The glaziers, however, made hand somely by the sale. The experiments here col „lected will show the facts in the case. (See Chemistry, Agricultural.)