CHAPTER II.

INFLUENCE OF LIGHT.-UPON VEGETATION.-UPON ANIMALS AND MAN. INJURIOUS CONSEQUENCES OF

DEFICIENCY OF THIS STI

MULUS.-LIGHT AND VISION.-SPIRITUAL ILLUMINATION.

THE Connexion between light and life, though but little understood as regards man and the higher animals, is nevertheless fully recognised and felt. Who is insensible to the exhilarating effects of a bright sunny day, after long-continued gloom; how the genial sunshine excites the animal spirits, imparting new energy for action, causing the heart to beat with more buoyancy, and animating even the spirit with brighter hope! The common experience of mankind has taught them to associate light with cheerfulness, movement, and the better qualities of our nature; while darkness has ever been considered emblematical of evil, despair, and death. But, though its importance and beneficial agency are thus recognised, in consequence of light being combined in its action with heat and other agents, and from the difficulty of experimenting upon man and the higher animals, it is almost impossible to determine the precise mode of its action in promoting the life and well-being of these. Some facts, it is true, have been acertained, bearing pretty directly upon the subject. More satisfactory conclusions, have however, been arrived at in reference to the action of light upon

plants and the simply organised tribes of animals, in consequence of the greater facility with which they may be submitted to philosophical experiment. A cursory review, then, of the effect of this vital agent upon the vegetable world and some of the lower animals, will better enable us to understand its sanitary influence upon man.

Light is intimately connected with the life of the plant, through the various stages of its growth to its fully matured state; stimulating the various functions upon which its existence depends, colouring its leaves, giving quality to its fruits, fixing its secretions, and playing a prominent part even in its decay. The germs, out of which spring the lower forms of vegetable life, would remain entirely undeveloped, if excluded from the sunshine. Professor Draper has shown that so long as spring-water is kept in the dark, it will remain clear and transparent, and that it is only on exposure that it gives indications of having previously contained the seeds of vitality. On being exposed, it soon becomes covered with a greenish tint which gradually increases, and which is found to be caused by vegetable structures of the simplest kind, called by botanists "confervæ," and consisting merely of cells. That the appearance of these little plants is due to the light and not to the heat is proved by the fact that water, exposed to the same degree of temperature in the dark, will exhibit no such effect; and that these plants are not then and there created that their germs were previously present in the air or water-is clearly shown by the circumstance, that distilled water exposed to the sun in air that has been care

fully freed from foreign particles, does not become covered with any green matter whatever. The process that takes place when ordinary spring water is thus placed in the sunshine, is easily explained. Plants in their young, as well as mature state, feed upon an elementary substance called "carbon." Now water always contains a certain quantity of carbonic acid, which consists of "carbon" and another element "6 oxygen." The germ or seed floating about in the water or air, being stimulated by the light, without which it would remain inactive, takes up the carbonic acid, and digesting it, retains the carbon and gives out the oxygen. Light is, however, injurious in the ordinary germination of seeds. In the first place it seems to exert a drying effect upon them, and in the next, ordinary seeds unlike our little conferva germ, contain all that is necessary for the nourishment of the plant in the early stage of its existence; light becoming essential only when the first leaves show themselves above the ground. From this time, the plant cannot well do without it. Its leaves and roots are going through much the same process as the germ did in the water. The spongy extremities of the roots take up the carbonic acid that is constantly being thrown out in the soil from decaying vegetable substances; they also take up water with various matters dissolved in it. These all circulate along the delicate sap-vessels until they reach the leaves, where the functions of digestion and respiration are carried on partly through the delicate tissue of the leaf, but principally through little mouths, technically called "stomata." The lips entering into the formation of these mouths, open widely

at the sun's bidding. Most of the water which the roots had absorbed from the soil, is given out in the form of vapour; while the carbonic acid, partly taken up by the roots, partly taken in from the atmosphere, becomes decomposed,-the oxygen being given out, and the carbon retained to form the green matter of the leaves, and the resins, oils, starches, and other products which distinguish different plants. At night, the exhalation of vapour, and the digestive process by which the carbonic acid is decomposed, cease; but respiration still goes on through the tissue of the leaves, chiefly from their under surface. But now, instead of exhaling oxygen, the leaves abs-tract it from the air, thus impairing the qualities of this for the purposes of animal life, and still further deteriorating it, by giving out carbonic acid. It appears, however, from the experiments of physiologists, that plants give out a much larger amount of oxygen than they consume, and thus play an important part in renewing the supply of this gas, which is of such vital importance to animals, and which is constantly being abstracted from the atmosphere by them as well as by the mineral world. Thus admirably is the balance maintained by an all-provident Creator!

Such being the influence of light upon plants, we have next to inquire, what may be the effects of its partial or complete withdrawal. One of the most striking effects of placing plants in partial or complete darkness is, that the evaporation of moisture is more or less arrested. The tissues of the plant become, in consequence, lax and soddened, or to use a medical term, dropsical. Horticulturists avail themselves of a know

ledge of this fact, when they wish to increase the succulence of a fruit or stem, and also place cut flowers or fruits in the dark, in order to keep them fresh by preventing evaporation. Another effect caused by deficiency of light is known to botanists as "etiolation," which in plain English means bleaching. If a growing plant be kept in the

dark, carbonic acid is constantly being given off by the leaves, and sufficient carbon is not retained to form the green colouring matter. The plant has a faded aspect, is pale and sickly in appearance, or, if the light be entirely excluded, becomes perfectly devoid of colour. Its form