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 accu-
