418 THE UNIVERSE. 



imprudently 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 maintain- 

 ing the harmonious comi^osition 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, 

 according to the calculations of M. Dumas, the atmos- 

 phere is so rich in oxygen that this event would not occur 

 till after a long series of ages. The learned chemist main- 

 tains that it would require at least 800,000 years for all 

 the animals on the globe to absorb the sum total of this 

 gas, and that 10,000 years would roll away without its 

 diminution being made sensible by our most jjerfect phy- 

 sical instruments.^ 



By means of ingenious investigations Professor Liebig 

 has even proved that the chemical nature of the atmos- 

 phere has not varied sensibly for upwards of 2000 years. 

 He took one of the little glass vases in which the Roman 

 ladies collected their tears, and Avhich, after being partly 



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

 copper, a kilometre (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 together would, in a centurj', only consume a 

 weight of oxygen equal to 15 or 16 cubic kilometres of copper, whilst the air 

 contains 134,000. 



It would require 10,000 years for all the people on earth to produce an effect 

 on the air appreciable by Volta's eudiometer (au instrument for measuring the 



