VEGETATION AND THE ATMOSPHERE. 179 
full, we invert it in a basin filled with water; if we then open the mouth the 
water will retain its elevation and continue to fill the inverted jar. The appa- 
ratus being thus arranged is to be carried to an open place where it can receive 
the direct rays of the sun. As soon as the light strikes the leaves of the im- 
mersed plant we see them become covered with a multitude of bubbles, which 
rapidly enlarge, unite and rise to the upper part of the jar, where they accu- 
mulate. Whenever the light is intereepted by the intervention of an opaque 
screen the disengagement of bubbles stops, and we can, at will, and even at a 
distance, by alternately intercepting the light and permitting it to strike the 
leaves, arrest or restore the production of the bubbles. At the end of some 
hours of continuous action the jar will be filled with gas, which resembles in 
ordinary appearance atmospheric air, but has not its properties, for if we intro- 
duce suddenly into the interior of the jar a small taper which has just- been 
extinguished, but which still retains at the extremity of its wick a few glowing 
points, it again instantly kindles and continues to burn with unwonted brillianey. 
The gas is not air, but oxygen. -In this form and with aquatic plants the 
experiment is striking, because the production of the gas in this case is rapid, 
and we assist, as it were, at the birth of the oxygen. We can produce the 
same effect, perhaps less rapidly, with all plants; and in grder not to change 
their ordinary condition we may expose them to the sun, under glass bells, pre- 
viously filled with carbonic acid; after the lapse of a day the carbonic acid gas 
will have disappeared and its place be supplied with nearly pure oxygen. 
Whatever may be the plant, or whatever be the experimental process employed, 
the action remains always the same. ‘The explanation of the tact is easy. The 
green part of the vegetable decomposes the carbonic acid, extracts the carbon, 
which it appropriates to itself, and abandons the oxygen to the atmosphere. 
In the dark, and during the night, the part performed is changed; then, in- 
stead of absorbing carbonic acid, the plant gives it off; but the nocturnal re- 
action being inferior to that of the day, the plant performs a part on the whole 
which is opposite to that of the animal—it absorbs the carbonic acid which the 
latter exhales, and returns to the atmosphere the oxygen which the animal 
consumed. 
Seeing the experiment so clearly, and its explanation so simple, it is difficult 
_ to conceive that they were not seen at the first glance. We find it difficult sto 
believe that this was not the case; but every great discovery is made at a cost 
to humanity. At the beginning all is obscurity and perplexity, and it is only 
after long research and after much hesitation that we settle upon a few scat- 
tered truths, and when a clear and steady light illuminates all the previous 
obscurity it is only after the labors of several generations are collected and the 
efforts of a succession of men of genius have been devoted to the object. The 
history of great discovery is not without interest, and I purpose in the follow- 
ing pages to retrace the several steps which have led to the establishment of 
the relations which exist between plants and the atmosphere, including in the 
sketch some of the results of the more recent investigations. » 
I. 
9 
Charles Bonnet, a Genevese physician, was the first who experimentally un- 
dertook, about the middle of the eighteenth century, the problem which occupies 
us. It was the perusal of a once celebrated work, The Spectacle of Nature, 
which had decided his vocation. His attention was first directed to the subject 
of spontaneous generation, a question agitated even at that period, and the inter- 
est of which has but augmented with the progress of time. He relinquished this 
subject to consider another whose fertility he did not perhaps altogether antici- 
pate; it was the inquiry, Of what use are leaves? and he made two experiments 
which have acquired a certain classic character. By the first, he proved that 
