PLANTS AND THE ATMOSPHERE. 427 



the animal consuming its nourishment would furnish carbonic acid to the plant, 

 acd the latter would restore to the animal the oxygen necessary for it. This 

 experiment would be a small model of the world, and it is thus that Priestley 

 understood its eternal equilibrium. Nothing is greater or more, beautiful than 

 this thought, but it requires completion. If the bell glass of which I have just 

 spoken were very small, the least excess arising in the respiration of the animal 

 or the least interruption in the action of the sun would so augment the quantity 

 of carbonic acid as to cause first the animal and afterwards the plant to perish. 

 Are we then exposed on the earth to a similar danger and are plants so necessary, 

 to us that we should cease to live as soon as they cease to act ? This cannot be, 

 and I am about to show that the fear of it is vain. The human population of the 

 globe may be approximately estimated at a thousand millions of inOTviduals, and 

 it would not be far from the truth to admit that all other animals taken together 

 exereise upon the atmosphere, by their respiration, an effect equal to three thou- 

 sand millions of adult men. That makes for the entire animal kingdom a popula- 

 tion equivalent to four thousand million human beings. The average quantity of 

 oxygen consumed by an adult daily, being measured, it would be easy to calculate 

 that consumed by the whole population of the globe. It is without doubt very 

 great, but, on the other hand, the supply of oxygen in the atmosphere is greater 

 still. It is so far beyond the consumption of animals that it would require eight 

 thousand million years to exhaust it. In eight centuries the thousandth part only 

 would be wanting, and, if plants were to cease their action, it would require at 

 least two thousand years for the most precise chemical analysis to succeed in 

 perceiving a change in the composition of the atmosphere. The service, therefore, 

 which plants render is much less immediate than Priestley thought ; it is a service 

 with a long date, and we may without ingratitude bequeath our thankful acknow- 

 ledgment to posterity. 



But the earth is very old and it is not impossible that its atmosphere may have 

 undergone since the ci'eation progressive changes, which by the long addition of 

 bygone ages must have become very considerable. This is a curious question 

 that has been treated of by M. Adolphe Brongniart and which we are now about 

 to study with him. The earth conceals enormous and so to speak inexhaustible 

 masses of carbon under the form of coal, anthracite, lignite and peat, and it cannot 

 be doubted, for a single moment, that these deposits are the accumulated fossil 

 remains of innumerable plants. Since there is but one way in which a plant caa 

 acquire carbon, that is, by taking it from the carbonic acid in the atmosphere, it 

 follows that all the masses of coal, which cover Belgium, England, and a great 

 part of America and which are found in every corner of the globe, were formerly 

 diffused throughout the atmosphere in a gaseous state ; they were there combined 

 with oxygen, and the globe at its creation was enveloped in an aeriform stratum 

 containing some nitrogen, a great quantity of carbonic acid and little or no 

 oxygen. Add to this the fact that at that period the earth was incandescent, and 

 it is manifest that all the carbon which it contained would certainly have been 

 consumed at this temperature by contact with oxygen. 



Thus constituted the earth cooled down ; but the composition of its atmosphere 

 made it uninhabitable by animals, since they require oxygen which it did not 



