QUANTITY OF OXYGEN IN THE ATMOSPHERE. 41 



statement, that the quantity of oxygen in the atmo- 

 sphere does not diminish in the course of ages,* — 

 that the air at the present day, for example, does 

 not contain less oxygen than that found in jars 

 buried for 1800 years in Pompeii, — appears quite 

 incomprehensible, unless some source exists whence 

 the oxygen abstracted is replaced. How does it 

 happen, then, that the proportion of oxygen in the 

 atmosphere is thus invariable ? 



The answer to this question depends upon another; 

 namely, what becomes of the carbonic acid, which is 

 produced during the respiration of animals, and by 

 the process of combustion? A cubic foot of oxygen 

 gas, by uniting with carbon so as to form carbonic 

 acid; does not change its volume. The billions of 

 cubic feet of oxygen extracted from the atmosphere, 

 produce the same number of billions of cubic feet 



* If the atmosphere possessed, in its whole extent, the same density 

 as it does on the surface of the sea, it would have a height of 24,555 

 Parisian feet; but it contains the vapor of water, so that we may as- 

 sume its height to be one geographical mile = 22,843 Parisian feet. Now 

 the radius of the earth is equal to 860 geographical miles ; hence the 

 Volume of the atmosphere = 9,307,500 cubic miles. 



= cube of 21 04 miles. 

 Volume of oxygen . . = 1,954,578 cubic miles. 



= cube of J 25 miles. 

 Volume of carbonic acid = 3,862-7 cubic miles. 



= cube of 15'7 miles. 



The maximum of the carbonic acid contained in the atmosphere has 

 not here been adopted, but the mean, which is equal to 0000415. (L.) The 

 weight of carbon which presses upon each square inch of the earth's 

 surface being 17*39 grains, on an acre of land will be 7 tons. — {Johnston.) 



A man daily consumes 45,000 cubic inches (Parisian). A man 

 yearly consumes 9505-2 cubic feet. 100 million men yearly consume 

 9,505,200,000,000 cubic feet. 



Hence a thousand million men yearly consume 0- 79745 cubic miles 

 of oxygen. But the air is rendered incapable of supporting the pro- 

 cess of respiration, when the quantity of its oxygen is decreased 12 

 per cent. ; so that a thousand million men would make the air unfit 

 for respiration in ^ million years. The consumption of oxygen by 

 animals, and by the process of combustion, is not introduced into the 

 calculation. 



When the air returns from the lungs, the carbonic acid gas amounts, 

 on an average, to 55th of the whole ; or its quantity is increased one 

 hundred times. — (Johnston.) A full grown man gives off from his 

 lungs, in the course of a year, upwards of 100 lbs. of carbon. It is 

 estimated by Johnston, that at least one third of the carbon of the 

 food of men is daily returned to the air, 



4* 



