Popular Science Monthly/Volume 2/January 1873/Light and Life

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LIGHT AND LIFE.
By FERNAND PAPILLON.
TRANSLATED FROM THE FRENCH BY A. R. MACDONOUGH, ESQ.

THE organized being that we observe on the surface of the globe does not subsist solely by the nourishment absorbed, sometimes in the form of aliment, sometimes in that of atmospheric air; it needs besides, heat, electricity, and light, which are like a secret and life-giving spring for the world. Its organs are subject to the twofold influence of an inner medium, represented by the humors moistening its tissues, and of an outer medium, composed of all those subtle and fluid agents with which space is filled. This close interdependence of beings and of the media in which they are immersed, too plain to have quite escaped notice, yet too complex for analysis by science in its infancy, has been brought in our day under piercing and methodical investigation, yielding results of remarkable interest. Light especially takes a part in this combination deserving deep study. Whether organic existence in its simplest expression and its lowest degree be considered, or whether we regard it in its highest functions, the influence of light upon it strikes us in the most strange and unlooked-for relations. Lovely forms and vivid colors, the hidden harmonies of life as well as its dazzling brightness and bloom, alike claim mysterious connection with that golden mist diffused by the sun over the world.

From this point of view, modern science finds reason in the simple worship paid by primitive man. It helps us to understand the divine honors given to the star of day among the earliest civilized nations, and the pathetic terror those child-like races suffered when, at evening, they saw the crimson globe, that was the source for them of all power and all splendor, slowly disappear in the horizon. That pious idolatry, far from being a mere utterance of gratitude for the wealth of fertility scattered by the sun over earth, was a homage, too, to the comforting source of brightness and joy, revealing the natural affinity between man and light. The Vedas, the Orphic hymns, and other remains of the earliest religions, are full of this feeling, which appears again in many poets and philosophers of antiquity, Lucretius and Pliny among others. Dante, invoking so often "the divine and piercing. light," crowns his poem by a hymn which more than any thing else is a symbolic description of the supreme brightness. On the other hand, laborers, gardeners, physicians, unite in bearing witness to the beneficial effects of light. Naturalists and philosophers, too, in all ages, impressed with the power of the sun, have described its manifold effects. Alexander Humboldt, following Goethe and Lavoisier, often notices its its various influences. Yet it was not until the middle of the eighteenth century that so rich a subject of study began to attract serious experimental research; and such are the difficulties of this grand and complex problem, that its solution is only partly revealed, in spite of a long series of attempts. Great deficiencies remain to be supplied, and many vaguely-known points to be cleared up, nor has an effort even been made as yet to systematize all the groups of results gained. The latter task we propose to attempt here, with the purpose of showing by a remarkable instance the manner of evolving knowledge through the power of the experimental method, the sequent, cumulative, and mutually-supporting character of well-conducted experiments, and their endless wealth of instruction; in a word, the process adopted by eminent men in the great art of wresting her secrets from living Nature.

 
I.

Plants gain their nourishment by the absorption through their roots of certain substances from the soil, and by the decomposition, through their green portions, of a particular gas contained in the atmosphere—carbonic-acid gas. They decompose this gas into carbon, which they assimilate, and oxygen, which they reject. Now, this phenomenon, which is the vegetable's mode of respiration, can only be accomplished with the assistance of solar light.

Charles Bonnet, of Geneva, who began his career by experimenting on plants, and left this attractive subject, to devote himself to philosophy, only in consequence of a serious affection of his sight, was the first to detect this joint work, about the middle of the eighteenth century. He remarked that vegetables grow vertically, and tend toward the sun, in whatever position the seed may have been planted in the earth. He proved the generality of the fact that, in dark places, plants always turn toward the point whence light comes. He discovered, too, that plants immersed in water release bubbles of gas under the influence of sunlight. In 1771, Priestley, in England, tried another experiment. He let a candle burn in a confined space till the light went out, that is until the contained air grew unfit for combustion. Then he placed the green parts of a fresh plant in the enclosure, and at the end of ten days the air had become sufficiently purified to permit the relighting of the candle. Thus he proved that plants replace gas made impure by combustion with a combustible gas; but he also observed that at certain times the reverse phenomenon seems to result. Ten years later, the Dutch physician, Ingenhousz, succeeded in explaining this apparent contradiction. "I had but just begun these experiments," says that skilful naturalist, "when a most interesting scene revealed itself to my eyes: I observed that not only do plants have the power of clearing impure air in six days or longer, as Priestley's experiments seem to point out, but that they discharge this important duty in a few hours, and in the most thorough way; that this singular operation is not due at all to vegetation, but to the effect of sunlight; that it does not begin until the sun has been some time above the horizon; that it ceases entirely during the darkness of night; that plants shaded by high buildings or by other plants do not complete this function, that is, they do not purify the air but that, on the contrary, they exhale an injurious atmosphere, and really shed poison into the air about us; that the production of pure air begins to diminish with the decline of day, and ceases completely at sunset; that all plants corrupt the surrounding air during the night, and that not all portions of the plant take part in the purification of the air, but only the leaves and green branches."

How do this transformation of impure air into pure air under the influence of sunlight, and the reverse process during darkness, take place? Senebier, the countryman and friend of Bonnet, gives us the answer. Applying to the problem the late discoveries of Lavoisier, he showed that the impure air absorbed and decomposed in the daytime by plants is nothing more than the carbonic acid thrown off by a burning candle or a breathing animal, and that the pure air which results from this decomposition is oxygen. He proved besides that the gas released by vegetables during the night is also carbonic acid, and consequently that the respiration of plants in the night-time is the reverse of that in the daytime. He also demonstrated that heat cannot supply the place of light in these processes. Thus the nature of the phenomenon was explained, but it remained to be learned what relation exists between the volume of carbonic acid absorbed and that of the oxygen released. Another Genevese, Theodore de Saussure, proved that the quantity of oxygen released is less than that of carbonic acid absorbed, and at the same time that a part of the oxygen retained by the plant is replaced by nitrogen thrown off; and supposed that this nitrogen was furnished by the substance of the plant itself. This function of the green portions of vegetables is, moreover, performed with great rapidity and energy. Boussingault, who has made some remarkable experiments on this subject, filled a vessel of water with vine-leaves, placed it in the sun, and sent a current of carbonic acid through it; on its passing out, he collected nothing but pure oxygen. It is calculated that a leaf of nenuphar gives out in this way during the summer more than 66 gallons of oxygen.

In 1848 Cloëz and Gratiolet contributed new facts. They showed that aquatic plants follow the same course during the day as others, but that at night they are at rest, and give rise to no release of carbonic acid. They proved the powerful, instantaneous action of solar light on vegetable respiration. If a few leaves of potamogeton or of nayas are put into a gauge full of water saturated with carbonic acid, as soon as the apparatus is placed in the sun, an immense number of light bubbles, of almost pure oxygen, are seen to detach themselves from the surface of the leaves. The shadow of a slight cloud, crossing the sky, suffices to check their disengagement at once, followed by sudden activity after it has passed. By intercepting the solar beam with a screen, the alternations of quickness and slowness in the production of gas-bubbles may be very plainly seen, according as the plant receives the rays or not. Water-plants show other interesting peculiarities. Diffused light has no power to excite the production of carbonic acid, unless the phenomenon has been first called forth by direct sunlight. Still further, the solar influence having once been applied, the evolution of carbonic acid continues even in darkness. The vegetable keeps up at night its mode of breathing by day. The living force of solar light, therefore, can be fixed and stored away in living plants, as Van Tieghem, the discoverer of this curious property, very well remarks, to act afterward in complete darkness, and exhaust itself by slow degrees, through transformation into equivalent chemical energy. It appears to lodge itself in phosphorescent sulphur, to reappear under the form of less intense radiations; it hoards itself up in paper, starch, and porcelain, to come forth anew, after a greater or less lapse of time, through its action on the salts of silver. The peculiarity residing in these green cells of vegetables, then, is not an isolated one: it is a special instance of the general property, inherent in many bodies, of retaining, within their mass, in some unknown form, a part of the vibrations that fall upon them, and of preserving them through transformation, to be afterward emitted, either in the state of luminous radiations, or in the condition of chemical or mechanical energy. The great principle of the transformation of forces thus holds good in the vegetable kingdom. And we end with the remark that these facts of persistent activity, called out by an initial excitement, lend support to the idea that living forces hold a close connection with the molecular structure of bodies, and may even be the determinate expression of that structure. We cannot conceive manifold energy in a mathematical and irreducible atom; but in a molecule, made up of a certain number of atoms, we can fancy dynamic figures of a very complex order.

We have thus far regarded only the action of white light, the effect of the totality of rays sent us by the sun; but this light is not simple. It is composed of a great number of radiations, of distinct colors and properties. When white light is decomposed by the prism, we obtain seven groups of visible rays, of unequal refractive power, violet, indigo, blue, green, yellow, orange, and red. The spectrum or ribbon of colors thus obtained widens and spreads out by invisible radiations. Beyond the red, there exist radiations of dark heat, or calorific rays, and, outside of the violet, radiations which are called chemical or ultraviolet rays. The first affect the thermometer, the last occasion energetic reactions in chemical compounds. What is their influence upon vegetation? Does solar light act by its colored rays, its heat-rays, or its chemical rays?

The question has been subjected to many important experiments, and is, perhaps, not yet determined. Daubeny, in 1836, was the first to watch the respiration of plants in colored glasses, and he found that the volume of oxygen released is always less in the colored rays than in white light. The orange rays appeared to him most energetic; the blue rays coming next. A few years later, Gardner, in Virginia, exposed young, feeble plants, from two to three inches long, to the different rays of the spectrum, and observed that they regained a green color with a maximum rapidity under the action of the yellow rays and those nearest them. In one of his experiments, green color was produced, under the yellow rays, in three hours and a half, under orange rays in four hours and a half, and under the blue, only after eighteen hours. Thus it is seen that the highest force of solar action corresponds neither with the maximum of heat, which is placed at the extremity of the red, nor with the maximum of chemical intensity, situated in the violet, at the other edge of the spectrum. Those radiations which are most active, from a chemical point of view, are the ones which have the least influence over the phenomena of vegetable life.

Mr. Draper, at present a professor in the New York University, and the author of a very remarkable history of the intellectual development of Europe, undertook new and more accurate experiments about the same time. He placed blades of grass in tubes filled with water which was charged with carbonic gas, and exposed these tubes, near each other, to the different rays of the solar spectrum. Then measuring the quantity of oxygen gas disengaged in each one of these little vessels, he proved that the largest production of gas occurred in the tubes exposed to the yellow and green light; the next, in the orange and red rays. In 1848, Cloëz and Gratiolet discovered the singular fact that the action of light on vegetation is more powerful when it passes through roughened glass than when transmitted through transparent glass. Julius Sachs, more lately, conceived the idea of measuring the degree of intensity of light-action, upon aquatic plants, by counting the number of gas-bubbles released by a cutting of a branch exposed to the sun in water charged with carbonic acid. He thus observed that the bubbles thrown off under the influence of orange light are very little less numerous than under white light, while the branch put under blue light throws out about twenty times less. These experiments are decisive. Neither the chemical nor the calorific rays of the solar beam act on plants. The luminous rays only, and chiefly the yellow and the orange, have that property. To these clearly-settled results, Cailletet added a new fact, that green light acts on vegetation in the same way as darkness. He assigns this reason for the feebleness of vegetation bathed in green light under the shade of large trees. It is true, this discovery of Cailletet has been warmly questioned recently, but it has found defenders too, Bert among others and we shall find soon that it harmonizes with the whole system of the actions of light in the two kingdoms of life.

A year ago, science had gone thus far, when a very distinguished botanist, Prillieux, published the result of a course of experiments made with an entirely different purpose, and taking up the study of the action of light from a new point of view. Resting on the twofold consideration that the distinctly-colored rays are not equally luminous, and that those of the greatest illuminating power are also those which act with most energy on plants, Prillieux undertook to examine what influence will be exercised on plants by rays different in color, but known to be equal in intensity, and whether this influence differs in the case of different colors, or is the same, provided they do not vary in illuminating power. The long and conscientious researches of this experimenter led him to the conclusion, that rays of different colors act with equal force on the green parts of plants, and produce an equal release of gas, when they have the like luminous intensity. He holds that all luminous rays effect the reduction of carbonic acid by vegetables in proportion to their illuminating power, whatever their refrangibility may be. If the yellow and orange rays are more active in this respect, it is because their luminous glare is much greater than that of the extreme rays.

The luminous rays also promote the production of green tissue, the green matter of all vegetables. Gardeners blanch certain plants by raising them in the dark. They thus obtain plants of a pale yellow, spindling, without strength or crispness. They are attacked by a true chlorosis, and waste away, as if sprung from barren sand. The sun also aids the transpiration of plants, and the constant renewal of healthy moisture in their tissues. On failure of the evaporation of moisture, the plant tends to grow dropsical, and its leaves fall, from weakness of the stem.

This love of plants for light, which is one of the most imperious needs of their existence, displays itself also in other interesting phenomena, which show that solar rays are, in very truth, the fertilizer that produces color. The corolla of vegetable species growing at great heights on mountains has livelier colors than that of species that spring in low spots. The sun's rays, in fact, pass more easily through the clear atmosphere that bathes high summits. The hue of certain flowers even varies according to the altitude. Thus the corolla of the Antliyllis vulneraria shades down from white to pale red and vivid purple. In general, the vegetation of open, well-lighted places is richer in color and development than that of regions not accessible to the sun. Some flowers originally white afterward deepen in color by the direct action of light. Thus Cheiranthus cameleo has a flower at first whitish, afterward yellow, and, at last, a violet-red. The Hibiscus mutabilis bears a flower which opens at morning with a white hue, and grows red during the day. The flower-buds of the Agapanthus umbellatus are white when they begin to unclose, and afterward take on a blue tint. If, at the moment of leaving its spathe, the flower is wrapped in black paper, intercepting the light, it remains white, but regains its color in the sun. The tints of fruits in the same way develop under the healthy action of daylight, and the rule extends to those principles of every nature which give taste and odor to the different parts of the the plant.

Flowers, fruits, and leaves, then, are elaborated by the help of luminous vibrations. Their tissue holds the sun's rays. Those charming colors, those fragrant perfumes, and delicious flavors, all the innocent pleasures the vegetable kingdom yields us, owe their creation to light. The subtle working of these wonderful operations eludes us, as does that which guides the fleeting diffusion and thousand-fold refractions displayed by the imposing spectacle of the dawn; but is it nothing to gain a glimpse of those primal laws, and to possess even a twilight ray upon these magnificent phenomena?

 
II.

Light exerts a mechanical influence on vegetables. The sleep of flowers, the bending of their stems, the nutation of heliotropic plants, the inter-cellular movements of chlorophyll, offer proofs of an extremely delicate sensitiveness of certain plants in this respect. Pliny mentions the plant called the sunflower, which always looks toward the sun, and steadily follows its motion. He notices, too, that the lupin always follows the sun in its daily movement, and points out the hour for laborers. Tessier, at the end of the last century, took up the study of these phenomena, and inferred in a general way that the stems of plants always turn toward the light, and bend over, if necessary, to receive it. He noted, too, that leaves tend to turn toward the side whence daylight comes. Payer made more exact experiments. He tried them with young stems of common garden cresses grown on damp cotton in the dark. These stems have the property of curving and turning quickly when placed in a room lighted only from one side or in a box receiving light on one wall only. The upper part of the stem curves first, the lower part remaining straight. By a second movement the top erects and the bottom bends over, so that the plant, though leaning, becomes almost rectilinear again. When put in a room receiving light from two windows, the following results are noticed: If the openings are on the same side admitting light equally, the stem bends in the direction of the middle of the angle formed by these two beams; if one of the two windows admits more light than the other, the stem leans toward it; if the windows are opposite each other, the stem stands erect, when light comes equally from both sides, and, if it does not, turns toward the stronger rays. Payer discovered, moreover, that the part of the irradiating light most active in its effects corresponds in this case to the violet and the blue. The red, orange, yellow, and green rays, do not seem to produce any movement in plants. Gardner carried the investigation still further. He sowed turnips, and let them develop in the dark to two or three inches in length. Then he threw the solar spectrum by a prism on this little field. The plants inclined toward a common axis. Those exposed to the red, orange, yellow, and green rays, leaned toward the deep blue, while the part lighted by violet bent in the opposite direction. Thus the crop took the appearance of a wheat-field bowing under two contrary winds. The turnips placed in the violet-blue region looked toward the prism. Gardner thus determined, as Payer had done, that the most refrangible rays are those which effect the bending of the young stems. He proved also that the plants grow erect again in the dark.

These experiments, repeated and varied in many ways by Dutrochet and Guillemin, uniformly gave like results, but the phenomenon itself still remains almost unexplained. This remark also applies to the very singular facts of the twisting of running plants. The stems of these plants, in twining about their supports, usually curl from the left to the right. Others follow the contrary course, and some twist indifferently in either way. Charles Darwin inferred, from his investigation, that light has an effect on this phenomenon. If twining plants are put in a room near a window, the tip of their stalk takes longer to complete the half circuit during which it turns toward the darker part of the room than that which is described nearer the window. Thus one of them, having gone through a whole turn in five hours and twenty minutes, the half circle toward the window employed a little less than an hour, while the other was not traversed in less than four hours and a half. Duchartre placed some China yams in full vegetation in a garden, and others in a completely dark cellar. The stems of the plants uniformly lost in the dark the power of twisting around their supporting sticks. Those exposed to the sun presented one portion twisting, but when put in the cellar they shot out straight stems. Yet some twining plants are known that seem to be independent of light in twisting.

The sleep of plants, in connection with light particularly, is still less understood. The flowers and leaves of certain vegetables droop and wither at fixed hours. The corolla closes, and after quiet inaction the plant again expands. In others, the corolla drops and dies without closing. In others still, as the convolvulus, the closing of the flower occurs only once, and its sleep marks its death. Linnæus noted the hours of opening and shutting in certain plants, and thus arranged what has been called Flora's clock; but the relations of these closings, with the intensity of light have not yet been scientifically determined.

The green coloring of vegetable leaves and stems is due to a special substance called chlorophyll, which forms microscopic granulations contained in the cells which make up these stems and leaves. These grains are more or less numerous in every cell, and it is their number as well as intensity of color that determines the tint of the plant's tissues. Sometimes they are closely pressed together, covering the whole inner surface of the cell; sometimes the quantity is smaller, and they are separate. Now, it has lately been discovered that in the latter case, under the influence of light, the green corpuscles we speak of undergo very singular changes of position. Some twelve years ago, Boehm noticed for the first time that in certain unctuous plants the grains of chlorophyll gather at one point of the wall of the cells under the action of the sun. He remarked that the phenomenon does not take place in the dark, nor in the red rays. The flat sheet made up of a single layer of cells, without epidermis, which composes the leaves of mosses, seemed to Famintzin the most suitable for this delicate kind of observations. He followed the movements, that take place in these sheets, by microscopic study. During the day the green coloring-grains are scattered about the upper and lower parts of the leaf-cells. At night, on the contrary, they accumulate toward the lateral walls. The blue rays affect them like white light; the yellow and the red ones keep the chlorophyll in the position it takes at night. The order of activity in the rays seems, then, to differ in this case from that in the phenomena of respiration. The researches of Borodine and Prillieux proved that these movements of coloring-corpuscles within the cells occur in almost all cryptogamous plants, and in a certain number of phanerogamous ones. The lately-published experiments of Roze show that in mosses the grains of chlorophyll are connected by very slender threads of plasma, and may suggest the idea that these threads are the cause of the changes of position just described. Perhaps there is some real relation between them; but it must not be forgotten that these movements of the plasmatic matter inside the cell take place by day and night, and that light has no marked effect on them. The green particles, on the contrary, creep over the walls of the cell, and move toward the lightest part as zoospores and some infusoria do.

Biot relates that in 1807, while at Formentera, employed in the work of extending the meridional arc, he devoted his leisure hours to the analysis of the gas contained in the swimming-bladder of fishes living at different depths in the sea. The oxygen required for these analyses was furnished him by the leaves of the cactus opuntia, which he exposed in water to sunlight, under hand-glasses, ingeniously applying the discovery of Ingenhousz and Senebier. It occurred to him one day to expose these leaves, in a dark place, to the illumination thrown by lamps placed in the focus of three large reflectors, used for night-signals in the great triangulation. He threw the light from three of these reflectors on the cactus-leaves. The eye, placed in this concentration of light, must have been struck blind, Biot says. The experiment, kept up for an hour, did not cause the release of a single gas-bubble. The glass was then taken into the diffused light outside the hut. The sun was not shining, but the evolution of gas took place at once with great rapidity. Biot is a little surprised at the result and concludes that artificial light is impotent to do what solar light can. The labors of Prillieux and other contemporary botanists have proved that all light acts on the respiration of plants, provided only it is not too powerful. In Biot's case artificial light had no effect, because it it was far too intense.

 
III.

Lavoisier somewhere says: "Organization, voluntary movement, life, exist only at the surface of the earth, in places exposed to light. One might say that the fable of Prometheus's torch was the expression of a philosophic truth that the ancients had not overlooked. Without light, Nature was without life; she was inanimate and dead. A benevolent God, bringing light, diffused over the earth's surface organization, feeling, and thought." These words are essentially true. All organic activity was very clearly at first borrowed from the sun, and if the earth has since stored away and made its own a quantity of energy, that sometimes suffices to produce of itself that which originally proceeded from solar stimulus, it must not be forgotten that those living forces, of startling and complex aspects, sometimes our pitiless enemies, often our docile servants, have descended, and are still descending upon our planet, from the inexhaustible sun. The study of animal life shows us by striking instances the physiological efficacy of light, and the immaterial chain, it may be called, which links existences with the fiery and abounding heart of the known universe.

In plants, as we have seen, respiration at night is the reverse of that by day. There are infusoria which behave, under the influence of light, exactly like the green portions of plants. These microscopic animalcula are developed in fine weather in stagnant water, and in breathing produce oxygen at the expense of the carbonic acid contained in the liquid. Morren saw that the oxygenation of the water occasioned by these little beings varied very perceptibly in the course of twenty-four hours. It is at the minimum at sunrise, and reaches its maximum toward four in the afternoon. If the sky is overcast, or the animalcula disappear, the phenomenon is suspended. This is only an exception. Animals breathe at night in the same way as in the daytime, only less energetically. Day and night they burn carbon within their tissues, and form carbonic acid, only the activity of the phenomenon is much greater in light than in darkness.

Light quickens vital movements in animals, especially the act of nutrition, and darkness checks them. This fact, long known and applied in practical agriculture, is expressly noted by Columella. He recommends the process of fattening fowls by rearing them in small dark cages. The laborer, to fatten his cattle, shuts them up in stables lighted by small low windows. In the half-light of these prisons the work of disassimilation goes on slowly, and the nutritive substances, instead of being consumed in the circulating fluid, more readily accumulate in the organs. In the same way, for the sake of developing enormous fat livers in geese, they are put into dark cellars, kept entirely quiet, and crammed with meal.

Animals waste away as plants do. The absence of light sometimes makes them lose vigor, sometimes entirely changes them, and modifies their organization in the way least favorable to the full exercise of their vital powers. Those that live in caverns are like plants growing in cellars. In certain underground lakes of Lower Carniola we find very singular reptiles resembling salamanders, called proteans. They are nearly white, and have only the rudiments of eyes. If exposed to light they seem to suffer, and their skin takes a color. It is very likely that these beings have not always lived in the darkness to which they are now confined, and that the prolonged absence of light has destroyed the color of their skins and their visual organs. Beings thus deprived of day are exposed to all the weaknesses and ill effects of chlorosis and impoverishment of the blood. They grow puffy, like the colorless mushroom, unconscious of the healthy contact of luminous radiance.

William Edwards, to whom science owes so many researches into the action of natural agents, studied, about 1820, the influence exercised by light on the development of animals. He placed frogs' eggs in two vessels filled with water, one of which was transparent, and the other made impermeable to light, by a covering of black paper. The eggs exposed to light developed regularly; those in the dark vessel yielded nothing but rudiments of embryos. Then he put tadpoles in large vessels, some transparent, others shielded from the light. The tadpoles that were shone upon, soon underwent the change into the adult form, while the others either continued in the tadpole condition, or passed into the state of perfect frogs with great difficulty. Thirty years later, Moleschott performed some hundreds of experiments in examining how light modifies the quantity of carbonic acid thrown off in respiration. Operating on frogs, he found that the volume of gas exhaled by daylight exceeds by one-fourth the volume thrown off in darkness. He established, in a general way, that the production of carbonic acid increases in proportion to the intensity of light. Tims, with an intensity represented by 3.27, he obtained 1 of carbonic acid, and, with an intensity of 7.38, he obtained 1.18. The same physiologist thinks that in batrachians the intensity of light is communicated partly by the skin, partly by the eyes.

Jules Béclard made more thorough researches. Common flies' eggs, taken from the same group, and placed at the same time under differently-colored glasses, all produce worms. But if the worms, hatched under the different glasses, are compared at the end of four or five days, perceptible differences may be seen among them. Those most developed correspond with the violet and blue ray; those hatched under the green ray are far less advanced; while the red, yellow, and white rays exert an intermediate action. A long series of experiments on birds satisfied Béclard that the quantity of carbonic acid thrown out in breathing, during a given time, is not sensibly modified by the different colors of the glasses the animals are placed under. It is the same with small mammifers, such as mice; but it is to be observed in this case that the skin is covered either with hair or feathers, and the light does not strike the surface. The same physiologist examined also the influence of the different-colored rays of the spectrum on frogs. Under the green ray, the same weight of frogs produces in the same period of time a greater quantity of carbonic acid than under the red ray. The difference maybe a half greater; it is usually a third or a fourth greater; but if the skin is afterward taken off the frogs, and they are replaced under the same conditions, the result alters. The amount of carbonic acid thrown out by the flayed frogs is greater in red than in green light. A few experiments tried by Béclard on the exhalation of the vapor of water by the skin show that in the dark, temperature and weight being alike, frogs lose by evaporation a half or a third less moisture than under white light. In the violet ray the quantity of moisture lost by the animal is perceptibly the same as in white light.

Light acts directly on the iris of almost all animals, and thus produces contraction of the pupil, while heat produces the reverse phenomena. This stimulus is observed in eyes that have been separated for some time from the body, as Brown-Séquard has shown.

Bert lately took up some very curious experiments on the preference of animals for differently-colored rays. He took some of those almost microscopic Crustacea, common enough in our fresh waters, the daphne-fleas, remarkable for their eager way of hurrying toward light. A number of these insects were put into a glass vessel, well darkened, and a spectrum of the ray then thrown into it. The daphnes were dispersed about the dark vessel. As soon as the spectrum-colors appeared, they began to move, and gathered in the course of the luminous track, but, when a screen was interposed, they scattered again. At first all the colors of the spectrum attracted them, but it was soon noticed that they hurried much more toward the yellow and green and even moved away a little if these rays were quickly replaced by the violet. In the yellow, green, and orange parts of the spectrum there was a thronging and remarkable attraction. A pretty large number of these little beings were remarked in the red, too, a certain number in the blue, and some, fewer in proportion to the distance, in the most refrangible portions of the violet and ultraviolet. For these insects, as for ourselves, the most luminous part of the spectrum was also the most agreeable. They behaved in it as a man would do who, if he wished to read in a spectrum thrown about him, would approach the yellow and avoid the violet. This proves, in the first place, that these insects see all the luminous rays that we see ourselves. Do they perceive the chlorific and chemic rays, that is to say, the ultra-red and ultra-violet ones, which do not affect our retina? Bert's experiments enable us to answer that they do not. That physiologist is even led to assert that, with regard to light and the different rays, all animals experience the same impressions that man does.

Let us now look at the influence of light upon the color of the skin in animals, noticing first the being which presents the strangest peculiarities in this respect, the chameleon. This animal, indeed, experiences very frequent modifications of color in the course of the same day. From Aristotle, who attributed these changes to a swelling of the skin, and Theophrastus, who assigned fear as their cause, to Wallisniéri, who supposes them to result from the movement of humors toward the surface of the animal's body, the most different opinions have been expressed on this subject. Milne-Edwards, thirty years ago, explained them by the successive inequalities in the proportions of the two substances, one yellowish and the other violet, which color the skin of the reptile, inequalities due to the changes in volume of the very flattened cells that contain these substances. Bruck, renewing these researches, proves that the chameleon's colors follow from the manifold dispersion of solar light in the colored cells, that is to say, from the production of the same phenomenon remarked in soap-bubbles and all very thin plates. Its colors, then, come from the play of sun-light among the yellow and violet substances distributed very curiously under its wrinkled skin. It passes from orange to yellow, from green to blue, through a series of wavering and rainbow-like shades, determined by the state of the light's radiation. Darkness blanches it, twilight gives it the most delicate marbled tints, the sun turns it dark. A part of the skin bruised or rubbed remains black, without growing white in the dark. Bruck satisfied himself, moreover, that temperature does not affect these phenomena.

All animals having fur or feathers are darker and more highly colored on the back than on the belly, and their colors are more intense in summer than in winter. Night-butterflies never have the vivid tints of those that fly by day, and among the latter those of spring have clearer, brighter shades than the autumn ones. The gold-and-azure dust that adorns them harmonizes with the tones of colors in surrounding Nature. Night-birds, in the same way, have dark plumage, and the downiness of their coverings contrasts with the stiffness of those that fly by day. Shells secluded under rocks wear pale shades, compared with those that drink in the light. We have spoken above of cave-animals. What a distinction between those of cold regions and those of equatorial countries! The coloring of birds, mammals, and reptiles, peopling the vast forests or dwelling on the banks of the great rivers in the torrid zone, is dazzling in its splendor. At the north we find gray tints, dead and of little variety, usually close upon white, by reason of the almost constant reflection from snow.

Not only the color of organized beings, but their shape too, is linked with the action of light, or rather of climate. The flora of the globe gain increasing perfection as we go from the poles toward the equator. The nearer these beings approach the highest degree of heat and light, the more lavishly are richness, splendor, and beauty, bestowed on them. The energy and glory of life, perfect forms as well as brilliant arraying, are the distinguishing mark of the various and manifold races in tropical regions, giving this privileged world its characteristic aspect. A pure emanation from the sun, Nature here lives wild and splendid, gazing unshrinkingly, like the Alpine eagle, on the eternal and sublime source which inundates it with heat and glow. Look, now, at the regions of the pole! A few dwarfish shrubs, a few stunted and herbaceous plants, compose all its flora. Its animals have a pale covering and downy feathers; its insects, sombre tints. All around them are the utmost limits of life—ice invades every thing, the sea alone still breeds a few acalephs, some zoophytes, and other low rudimentary organizations. The sun comes aslant and seldom. At the equator he darts his fires, and gives himself without stint to the happy Eden of his predilection.

 
IV.

It remains to note the relations of light to that being most sensitive to its influence, and best able to express its effects, man himself. The newborn child seeks the day by instinct, and turns to the side whence light comes, and, if this spontaneous movement of the infant's eyes is thwarted, strabismus may be the consequence.

Of all our organs the eye is the one that light especially affects. Through the eyes come all direct notions of the outer world, and all impressions of an æsthetic kind. Now, the excitability of the retina shows variations of every kind. Prisoners confined in dark cells have been known to acquire the power of seeing distinctly in them, while their eyes also become sensitive to the slightest changes in the intensity of light. In 1766 Lavoisier, in studying certain questions upon the lighting of Paris, which had been given for competition by the Academy of Sciences, found after several attempts that his sight wanted the necessary sensitiveness for observing the relative intensities of the different flames he wished to compare. He had a room hung with black, and shut himself up in it for six weeks in utter darkness. At the end of that time his sensitiveness of sight was such that he could distinguish the faintest differences. It is very dangerous, too, to pass suddenly from a dark place into a strong flood of light. The tyrant Dionysius had a building made with bright, whitewashed walls, and would order wretches, after long seclusion from light, to be suddenly brought into it. The contrast struck them blind. Xenophon relates that many Greek soldiers lost their sight from reflections off the snow in crossing the mountains of Armenia. All travellers who have visited the polar regions have often seen like results produced by the glare of the snow. When the impression of light on the eye is sudden and overpowering, the retina suffers most. If it is less powerful, but longer continued, the humors of the eye are affected. The phenomenon called sunstroke results from the action of light, and not, as is often supposed, from excessively high temperature. It sometimes occurs in the moderately warm season of spring; or a very intense artificial light, and particularly the electric light, may occasion it. The violet and ultra violet parts of the sunbeam seem to be the cause of this action, for screens of uranium glass, that absorb these portions, protect the eyes of experimenters occupied in studying the electric light. This disorder is a true inflammation.

The action of light on the human skin is manifest. It browns and tans the teguments, by calling out the production of the coloring-matters they contain. The parts of the body usually bare, as the skin of the face and hands, are darker than others. In the same region, country-people are more tanned than town residents. In latitudes not far apart, the inhabitants of the same country vary in complexion in a measure perceptibly related to the intensity of solar light. In Europe three varieties of color in the skin are distinctly marked: olive-brown, with black hair, beard, and eyes; chestnut, with tawny beard and bluish eyes; blond, with fair, light beard and sky-blue eyes. White skins show more readily alterations occasioned by light and heat; but, though less striking, facts of variation in color are observable in others. The Scytho-Arabic race has but half its representatives in Europe and Central Asia, while the remainder passes down to the Indian Ocean, continuing to show the gradual rising heat of climate by deepening brown complexions. The Himalayan Hindoos are almost white; those of the Deccan, of Coromandel, Malabar, and Ceylon, are darker than some negro tribes. The Arabs, olive and almost fair in Armenia and Syria, are deep brown in Yemen and Muscat. The Egyptians, as we go from the mouths of the Nile upstream toward its source, present an ascending chromatic scale, from white to black, and the same is true of the Tuariks on the southern side of Mount Atlas, who are only light-olive, while their brethren in the interior of Africa are black. The ancient monuments of Egypt show us a fact equally significant. The men are always depicted of a reddish brown; they lived in the open air, while the women, kept shut up, have a pale-yellow complexion. Barrow asserts that the Mantchoo Tartars have grown whiter during their abode in China. Rémusat, Pallas, and Gutzlaff, speak of the Chinese women as remarkable for a European fairness. The Jewesses of Cairo or Syria, always hidden under veils or in their houses, have a pallid, dead color. In the yellow races of the Sumatra Sound and the Maldives, the women, always covered up, are pale like wax. We know, too, that the Esquimaux bleach during their long winter. These phenomena, no doubt, are the results of several incluences acting at once, and light does not play the sole part in them. Heat and other conditions of the medium probably have a share in these operations of color. Still, the peculiar and powerful effect of luminous radiation as a part of them is beyond dispute.

The whole system of organic functions shares in the benefits of light. Darkness seems to favor the preponderance of the lymphatic system, a susceptibility to catarrh in the mucous membranes, flaccidity of the soft parts, swellings and distortions of the bony system, etc. Miners and workmen employed in ill-lighted shops are exposed to all these causes of physiological suffering. We may notice, with regard to this, that certain rays of the solar beam affect animals like darkness; among others, the orange light, which, according to Bert, hurts the development of batrachians. Now, if this light is injurious to animals, it is not so to plants, as we have seen. In exchange, green light, which is hurtful to vegetables, is extremely favorable to animals. There is a kind of opposition and balance, then, as respects luminous affinities, between the two great kingdoms of life. White light, as Dubrianfant says, seems to split up under the influence of living beings into two complementary groups, a green group and an orange group, which exhibit in Nature antagonistic properties. It is quite certain that green light is a very lively and healthful stimulant for our functions, and that, for that reason, spring is the favored and enchanted season.

The correspondence between perfection of forms and heightening of luminous intensity proves true in the human race as in others. Æsthetics, agreeing with ethnography, demonstrate that light tends to develop the different parts of the body in true and harmonious proportion. Humboldt, that nice observer, says, speaking of the Chaymas: "The men and women have very muscular bodies, but plump, with rounded forms. It is needless to add that I have never seen a single one with any natural deformity. I will say the same of so many thousands of Caribs, Muycas, Mexican and Peruvian Indians, whom we have observed during five years. These bodily deformities and mis-growths are extremely rare in certain races of men, especially among people who have a deep-colored skin." No doubt there is great difficulty in conceiving how light can model—can exert a plastic power. Yet, reflecting on its tonic effect on the outer tegument, and its general influence over the functions, we may assign it the part of distributing the vital movement orderly and harmoniously throughout the whole of the organs. Men who live naked are in a perpetual bath of light. None of the parts of their bodies are withdrawn from the vivifying action of solar radiation. Thence follows an equilibrium which secures regularity in function and development.

It is commonly said that an ordained causality rules the operations of matter, and that free spontaneity is the privilege of those of spirit. It might well be said on this subject that, in many cases, the causes acting in matter elude us, and, not less often, the causes which act in spirit overpower us; but it is not our task to elucidate that terrible antithesis of law, when the genius of Kant failed in it. We would only ask it to be observed how great an influence light has on the system of the intellectual functions. The soul finds in it the least deceiving of the consolations it seeks for the eternal sadness of our destiny — the bitter melancholy of life. Thought, fettered and dumb in a dark place, springs into freedom and spirit at evening, in a room brilliant with light. We cannot shun the sad moods caused by gloomy and rainy weather, nor resist the impulse of joy given by the spectacle of a brilliant day. Here we must confess our slavery — yet a slavery to be welcomed, that yields only delights. And why should we not join in the chorus of all animate and inanimate things, which, at the touch of light, quiver, and thrill, and betray in a thousand languages the magical, rapturous stimulus of that contact? By instinct, and spontaneously, we seek it everywhere, always happiest when it is found. In some sort, it suffices us. And what a part it plays, what a charm it gives, in works of poetry and art!

This is not the place to unfold that attractive and hardly-opened chapter of æsthetics — to demonstrate the relation between the atmosphere and art, by interrogating the climates of the globe and the great masters of all ages, not following a system of empirical analogies and far-fetched suggestions, but led by strict physiology and rigid optic laws. A charming picture would unfold in tracing the countless and changeful aspects of the sky, and all the caprices of light and air in their influence over the moral and physical nature of painters, poets, and musicians. The ever-varying face of the sun, the fires of dawn and sunset, the opalescent play of air, the shimmer of twilight, the blue, green, shifting hues and iridescent gleams of sea or mountain — all these things find a destined answer in the inmost and unconscious ongrowings of life, as in the soul of one who looks understandingly at Nature's works. In it they reveal and transform themselves by subtlest thrills — tender and creative. He who shall detect these — shall link, range, and embrace them in their wonderfully complex unity — will render a great service to science and to art. He will not make the artist an automaton, nor prove man the copy of a plant, drawing all its virtues from the soil it springs in, but he will lay his hand upon the mechanism, as yet scarcely guessed, moving a whole system of mighty combinations of energy.—Revue des Deux Mondes.