VEGETATION AND THE ATMOSPHERE. 185 
whose function it is to decompose carbonic acid. These explanations have not 
only the defect of being illusory and conjectural, they are demonstrably false, 
‘for the pounded leaves, which preserve the same composition, ought then to 
continue the same functions, which is not true. There is also a whole school 
of naturalists who content themselves with ascribing the functions of vegeta- 
bles to what they call life—a kind of unapproachable foree which suffices to 
explain everything by the sole virtue of its name; these appear to me to re- 
nounce all of scientitic progress, like the ignorant devotees who explain all 
phenomena by saying that God produces them. It is God, beyond a doubt, 
who has ordered the world, but he permits us sometimes to inspect the me- 
chanism. Without doubt, also, it is life which disposes the functions of beings, 
but before proposing it as the final cause and ultimate explanation of facts, it 
behooves to know a little what life is, and of what contrivances it makes nse. 
We see to what weakness we are reduced as soon as the basis of experiment 
fails us, when, in order to fill up the gaps of our knowledge, we strive to grap- 
ple ourselves to hypotheses, to unexplained forces which explain nothing. Let 
us honestly avow our ignorance, and gird up our loins and seek. 
T’o console ourselves for this avowal, which may possibly hurt our self-love, 
and to find encouragement for the labors of to-morrow, let us measure, with a 
view to their consequences, the importance of the facts which we know to-day. 
If plants give out oxygen, animals absorb it, and a compensation is established 
between these inverse functions. We can demonstrate it experimentally by 
confining under a glass bell an animal anda plant. Separated, each of them 
would die—the first by being suffocated in the carbonic acid it exhales, the 
second because it would be deprived of this gas which nourishes it. United 
in the dark, the animal and the vegetable would injure instead of aiding one 
another; but in the light of the sun the life of the one supports that of the 
other; the animal, burning his aliments, furnishes carbonic acid to the plant, 
and the latter restores to the animal the oxygen which is necessary to it. This 
experiment would be in little the image of the world, and it is thus that Priestley 
- conceived the eternal equilibrium of it. Nothing can be more grand and 
beautiful than this thought, but it is necessary to complete it. If the bell of 
which we have just spoken were very small, the least excess which might occur 
in the respiration of the animal, or the least interruption in the action of the 
sun, would exaggerate the quantity of carbonic acid and cause first the animal 
to perish, and then the vegetable. Are we, then, exposed on the earth to alike 
danger, and are plants so necessary to us that we must cease to live as soon as 
they should cease to act? Believe it not, for we shall demonstrate that this 
fear would be -vain. The human population of the globe may be approxi- 
mately rated at a thousand millions of individuals, and we shall not be far 
from the truth in assuming that all other animals taken together exert upon 
the atmosphere, by their respiration, an effect equal to that of three thousand 
millions of adult men. This makes for the whole animal kingdom a population 
equivalent to four thousand millions of human beings. Now, as the mean 
quantity of oxygen which an adult human being consumes in a day has been 
measured, we can calculate that of the total population of the globe. It is 
very great, no doubt, but, on the other hand, the provision of oxygen in the 
atmosphere is greater still. It is so much greater than the consumption of 
animals that it would require eight thousand millions of years to exhaust it. 
In eight centuries it would fail but a millionth part, and if the vegetables 
ceased their action, it would require at least two thousand years before the 
nicest chemical analysis could avail to detect a change in the composition of 
the air. The service that vegetables render us is therefore much less imme- 
diate than Priestley thought; it is a service of distant reversion, and we may 
without ingratitude relegate our acknowledgments to posterity. 
