FERTILITY OF DIFFERENT SOILS. 13 



with that of the origin of humus. It is universally admitted 

 that humus arises from the decay of plants. No primitive 

 humus, therefore, can have existed — for plants must have pre 

 ceded the humus. 



Now, whence did the first vegetables derive their carbon ? and 

 in what form is the carbon contained in the atmosphere ? 



These two questions involve the consideration of two most 

 remarkable natural phenomena, which, by their reciprocal and 

 uninterrupted influence, maintain the life of individual animals 

 and vegetables, and the continued existence of both kingdoms of 

 organic nature. 



One of these questions is connected with the invariable con- 

 dition of the air with respect to oxygen. One hundred volumes 

 of air have been found, at every period and in every climate, to 

 contain 21 volumes of oxygen, with such small deviations that 

 they must be ascribed to errors of observation. 



Although the absolute quantity of oxygen contained in the 

 atmosphere appears very great when represented by numbers, 

 yet it is not inexhaustible. One man consumes by respiration 

 25 cubic feet of oxygen in 24 hours ; 10 cwt. of charcoal con- 

 sume 32,066 cubic feet of oxygen during its combustion, so that 

 a single iron furnace consumes annually hundreds of millions of 

 cubic feet ; and a small town like Giessen (with about 7000 in- 

 habitants) exacts yearly from the air, by the wood employed as 

 fuel, more than 551 millions of cubic feet of this gas. 



When we consider facts such as these, our former statement, 

 that the quantity of oxygen in the atmosphere does not diminish 

 in the course of ages* — that the air at the present day, for exam- 



* 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; hut it contains the vapor of water, so that we may assume its height 

 to be one geographical mile=22,8<-13 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. 

 Volume of oxygen . . =1,954,578 * 

 Volume of carbonic acid =3,8G27 " 



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



