Page:Popular Science Monthly Volume 6.djvu/75

Rh acid. But we have still to inquire why it is that the exhalation of carbonic acid gradually diminishes as the leaves grow in size. This is the great point to settle. Inasmuch as the respiratory organ grows in vitality and in size, it looks as though the respiration ought to become more active, and consequently augment the exhalation of carbonic acid, if this latter process is the respiration.

In order to solve this problem, M. Corenwinder judged it necessary to investigate very closely the variations occurring in the chemical composition of leaves during their vegetation. For this purpose he has made numerous researches, whereof we will describe two experiments made during the summer of 1873, one upon a white lilac, the other upon a maple with fine green leaves. These occupy a good, airy site in the author's garden, near the city of Lille.

M. Corenwinder gathered leaves of these plants at suitable intervals, from April 15th till October 31st, analyzing them afterward to determine the amount of water, nitrogenized substances, ash, and ternary compounds, they respectively contained. In sundry cases, at the most characteristic periods, the proportion of phosphoric acid contained in the ash was accurately weighed.

As every one knows, water forms a considerable portion of the substance of leaves, as much as four-fifths. As a rule, this proportion becomes less as the season advances, and the leaves grow older, but the diminution is not regular. As M. Corenwinder has shown, it needs but to rain for a little while to very sensibly raise again the proportion of water in leaves. These variations in the water of vegetation of leaves make it difficult to compare the other elements which they contain, and hide the relative increase or decrease of each of these elements. De Saussure evaded this difficulty by calculating the leaves in the dry state, and then determining what would be the relative proportions per cent, of the various elements in each leaf, if really deprived of all its water. M. Corenwinder adopts the same course. Having given in full the results of his two series of experiments, he condenses them in the following tables, which enable us easily to follow the evolution of each of the groups of elements in the leaf: