516 
On the Food of Plants. 
rose to 25. At night it sunk to nearly 22 per cent., but the next 
evening it had again risen to 2*7. This was tlic maximum of its in- 
crease ; for at night it sunk to 26, and in the morning exhibited signs 
of incipient decay. Accordingly, in the evening, the oxygen only 
amounted to 26 " 5, the next evening to 25 ■ 5, the following one to 24 • 15, 
and the one next succeeding it had fallen to the point at which it stood 
at the commencement, or 21 per cent. The reason of this decrease was, 
however, very manifest from the. decay and falling oft' of the leaves; so 
that this circumstance does not invalidate the conclusion which the pre- 
ceding experiments concur in establishing, namely, that in fine weather 
a plant, so long at least as il 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 in- 
closed in the jar, about 50 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. But there is 
reason to believe that, even imder the circumstances above stated, the 
amount of oxygen evolved was much smaller than it would be in the 
open air; for I have succeeded, by introducing several plants into the 
same jar of air in pretty quick succession, in raising the amount of 
oxygen contained from 21 to 39 per cent., and probably had not even 
then attained the limit to which the increase of this constituent might 
have been brought."* 
These experiments are admirable. There is one point, how- 
ever, which Dr. Daubeny does not notice, namely, the alteration 
in the whole volume of the gas by the plant, connected with a 
very curious observation of De Saussure — the exhalation of 
nitrogen. It appears that something of this kind occurs in ani- 
mals, and it is a subject worth examining. To take into account, 
however, any change of volume of the air in a large jar would 
involve certain corrections at each time of observing, which would 
materially increase the labour attending such experiments. 
Such is the nature of the experimental evidence in favour of 
the supposition. We need not, however, confine ourselves to 
watching the growth of a plant in a glass jar. The observation 
of the great phenomenas of nature seems indisputably to lead to 
this conclusion : — 
" The mere observation of a wood or a meadow is infinitely better 
adapted to decide so simple a question than all the trivial experiments 
under a glass globe : tlie only difference is, that instead of one plant 
there are thousands. When we are acquainted with the nature of a 
single cubic inch of the soil, and know the composition of the air and 
rain-water, we are in possession of all the conditions necessary to their 
life. The source of the different elements entering into the composition 
of plants cannot possibly escape us, if we know in what form they take 
* Phil. Trans, for 1836. See, also, Three Lectures on Agriculture. 
Murray. 1841. 
