Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy.

  • Humphry Davy
Date:
1846
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Licence: Public Domain Mark

Credit: Elements of agricultural chemistry, in a course of lectures of the Board of Agriculture / By Sir Humphry Davy. Source: Wellcome Collection.

    189/318 (page 173)
    rapidly destroy the oxygen of the air. In some of the early experiments of Dr. Priestly before he was acquainted with the agency of light upon leaves, air that had supported combustion and respiration, was found purified by the growth of plants when they were exposed in it for succes- sive days and nights; and his experiments are the more unexceptionable, as the plants, in many of them, grew in their natural states; and shoots, or branches from them, only were introduced through water into the con- fined atmosphere. * * The opinion stated in the text that Ellis’s experiments were not so conducted that the accuracy of the results could be relied on, has been completely borne out by the more recent and accurate results of Dr. Daubeny. By operating with large vessels, and particularly by removing the plants as soon as they be- came sickly, he has shewn, “ that plants, even in a confined atmosphere, do in reality add a great deal more oxygen to the air than they abstract from it, whilst the amount of carbonic acid which may be introduced undergoes at the same time a corresponding diminution,” and this effect he “ even found to take place in diffused light, as well as under the direct influence of the solar rays, and to be no less common in aquatic than in terrestrial plants.” And further, “ that when a branch loaded with flowers, as well as leaves, was introduced into a jar containing a certain proportion of carbonic acid, the balance still continued to be in favour of the purifying influence of the vegetable.’’ The following account of one of his most successful experiments is remarkably instructive:—“ In a second trial, however, the branch of a healthy lilac growing in the garden was introduced into the same jar, where it was suffered to remain until its leaves were entirely withered. The first day the increase of oxygen in the jar was no more than 0*25 per cent., but on the second it rose to 25'0. At night it sunk to nearly 22*0 per cent., but the next evening it had again risen to 27*0. This was the maximum of its increase, for at night it sunk to 26’0, and in the morn- ing exhibited signs of incipient decay. Accordingly, in the evening, the oxygen amounted only to 26*5; and the next evening to 25*5; the following one to 24*75; and the one next succeeding it had fallen to the point at which it stood at the commencement, or to 21*0 per cent. The reason of this decrease was, how- ever, very manifest from the decay and falling off of the leaves; so that this circumstance does not invalidate the conclusion which the preceding experiments concur in establishing, namely, that in fine weather a plant, so long at least as it continues healthy, adds considerably to the oxygen of the air when carbonic acid is freely supplied. In the last instance quoted, the exposed surface of all the leaves inclosed in the jar, which were about fifty in number, was calculated at not more than 300 square inches, and yet there must have been added to the air of the jar as much as 26 cubic inches of oxygen, in consequence of the action of this surface upon the carbonic acid introduced. [The carbonic acid was every morning made equivalent to five or six per cent.] But there is reason to believe, that even under the circumstances above stated (which appear more favourable to the due performance of the functions of life than those to which Mr Ellis’s plants were subjected,) the amount of oxygen evolved was much smaller than it would have been in the open air, for I have succeeded by intro- ducing several plants into the same jar of air in pretty quick succession, in raising the amount of oxygen contained from twenty-one to thirty-nine per cent., and probably had not even then attained the limit to which the increase of this con- stituent might have been brought. How great then must be the effect of an entire tree in the open air under favourable circumstances! and we must recol- lect that, cateris paribus, the circumstances will be favourable to the exertion of the vital energies of the plant, within certain limits at least, in proportion as animal respiration and animal putrefaction furnish to it a supply of carbonic acid ”— Three Lectures on Agriculture. But apart from all eudiometric investigations, the simplest, and, at the same time, the most satisfactory proof of these two facts, that under the influence of