390 THE UNIVERSE. 



But during the night the respiratory phenomena of plants 

 take the very opposite direction ; then they act like ani- 

 mals. They absorb the vital part of the air and exude car- 

 bonic acid by all their pores, to such an extent that if we 

 sleep in a close chamber, in which shrubs have been impru- 

 dently left, the air is as much vitiated by them as if it had 

 contained an equal number of men. 



But this nocturnal respiration is far from neutralizing the 

 benefit effected by the diurnal exhalation. Plants under 

 the influence of light pour into the atmosphere much more 

 oxygen than they absorb by night, and they withdraw from 

 it greatly more carbonic acid every day than they produce 

 during darkness. 



It is to the plant, therefore, that the task of maintaining 

 the harmonious composition of the air is intrusted. It is 

 evident that were the important function confided to plants 

 to be suddenly annihilated, all the animal kingdom would 

 within a given time succumb in its turn. However, accord- 

 ing to the calculations of M. Dumas, the atmosphere is so 

 rich in oxygen that this event would not occur till after a 

 long series of ages. The learned chemist maintains that it 

 would require at least 800,000 years for all the animals on 

 the globe to absorb the whole of this gas, and that 10,000 

 years would pass without its diminution being made sensi- 

 ble by our most perfect instruments. 1 



1 The weight of the air which encircles us is equal to 581,000 cubes of copper, 

 1093 yards on every side. The oxygen in it weighs as much as 134,000 of these 

 cubes. Supposing the earth to be peopled by 1,000,000,000 souls, and taking the 

 animals on it as equivalent to 3,000,000,000 men, we should find that these to- 

 gether would, in a century, only consume a weight of oxygen equal to 15 or 16 of 

 these cubes of copper, whilst the air contains 134,000. 



