CHAP. X.

Light.

It is difficult to separate light from caloric, particularly in the rays of the sun; yet it may, perchance happen, and give rise to very curious phenomena. It is principally to the effect of light on the development of man, that we mean, at present, to confine our attention.

Light has different degrees of influence on animal economy, vegetation, and inorganized bodies. In the animal kingdom, solar rays produce effects that could not possibly take place in obscurity; the verdure of plants is owing to the influence of light; and it may be said, that man's colour or complexion is due to the same cause. Plants growing in shade or darkness are etiolées, lose their properties and consistence; increase in length, but are slender and weak. Human beings are equally dependent on the influence of light. If man be confined in a dungeon, or in any habitation where the rays of the sun do not penetrate, in cellars or mines, for example, his whole complexion turns sallow; his strength fails, and aqueous humours break out on his skin: persons who have been thus deprived of light become languid, and frequently dropsical. If the absence of light has so strong an influence on

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adults, let us examine its operation on young animals.

The experiments made by Edwards are extremely interesting; he says that before animals come in contact with the exterior world, they have grown in darkness, and he has endeavoured to discover what influence light possesses independent of heat. To effect this purpose, he placed a certain number of frogs in two separate vases, filled with water; the one carefully wrapped up with an outside covering of black paper, the other left transparent; they were both placed in the same temperature; the light shining on the transparent vase, the eggs it contained were all hatched, while those in the dark were not so.

But even were it possible, that under more favourable circumstances, the development could take place without light, the experiment just related, proves how fully this agent contributes to growth.

It is principally after birth, that it is interesting to discover the full effects of light on the development of the body, as all animals are more or less exposed to it; yet, it is difficult, although with growth their shape be changed, precisely to remark and appreciate the modifications, unless we select a species among the vertebrated class, whose development offers a precise and palpable difference.

Frogs are admirably suited for these experi

ments; in the first instance, they are like fishes, having no limbs, but a tail and fins; in their second stage, they are reptiles, bearing no resemblance to fish; they have four limbs, no tail, no fins; the metamorphosis is complete. Changes so marked, offered to the experimentalists the hope of being enabled to settle a point so important to science, and the following is the result.

In order to continue his experiments, Edwards used a tin box, made expressly for the purpose, divided into twelve parts, each being numbered, and pierced with small holes, so as to admit the water; then, taking some tadpoles, he weighed them, and placed one in each of the twelve compartments; the box was put into the river, a few feet under water.

Several more of these animals were thrown into an earthen pan, containing thirty pounds of water, from the Seine, which was changed every day. These tadpoles could reach the surface of the water, and breathe the fresh air; and they shortly appeared in the most perfect form; whilst, of those in the box, under water, ten out of twelve underwent no change, though they were considerably grown; some were twice, others three times as heavy as when first shut up; besides which, when the experiment was begun, they were already near the period when the metamorphosis takes place: two were transformed, but at a much later period than those exposed to light and air.

These experiments clearly prove, that the

want of light does not absolutely prevent the growth of batraciens; though only two, out of twelve, kept in darknes, were metamorphosed; while all those exposed to light and air, underwent the usual transformation.

Reflecting upon these facts, says Edwards, it is evident, that the action of light aids the development of the different parts of the body, in that just proportion which constitutes the type of the species: this type is only characterized in the adult age. These changes are more striking in young subjects, and in the species used in the experiment just cited, the transition is most remarkable.

The principle deduced from the experiments on animals, naturally leads to the following considerations, relative to man, in climates where nakedness is not incompatible with health.

The exposition of the whole body to light, is very favorable to its regular conformation; a fact corroborated by the observation of Humbolt, in a voyage to the equinoctial regions.* Speaking of the Chaymas, he observes, "both men and women, are muscular, though thin. I saw no sign of any natural deformity. I could say the same of thousands of Caribs, Indians, Mexicans, and Peruvians, whom in five years, I had full opportunity of observing. Deformity and distortions of the body, are rare among a certain

* Paris, 1814, 4to. p. 147.

race of men, particularly with those who have the dermoid system, strongly coloured. I cannot think they depend entirely on the progress of civilization, effeminacy of life, and corrupted manners; but whatever may be the multiplicity of causes that produces them, we cannot doubt that a deficiency of the action of light upon the surface of the body contributes to these distortions of shape in the soft and indurated parts of children affected with scrofula, mostly found among the poor, who inhabit narrow dark lanes."

"It follows, from the same principle," continues Edwards, " that when these deviations do not appear incurable, exposure to the sun, in open air, is one of the means most likely to aid recovery. Thus, the influence of light on different organized bodies, merits the greatest attention. In subterraneous caverns, if there be any vegetation, it is confined to moss. When plants are kept in the dark, they lose their shape and flavour; the growth of animals is arrested, when they are deprived of light; and if left in obscurity, for any length of time, the human frame becomes quite altered.

In some individuals, there is a predominance of lymphatic and liquid vessels; complete atony is spread on the surface of the body; not only in individuals buried in mines and caves, but this paleness is also observable in females of delicate constitutions, who, through fear of destroying their complexion, sedulously avoid broad day light, instead of seeking the genial heat of the