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
