QUANTITY OF OXYGEN IN THE ATMOSPHERE. IC 



volume as from one-half to ten times greater than that which it 

 has at the present time : so that we can hence at most conclude 

 that it has diminished. 



It is quite evident that the invariable quantities of carbonic acid 

 and oxygen in the atmosphere, must stand in some fixed relation 

 to one another ; a cause must exist which prevents the increase 

 of carbonic acid by removing that which is constantly forming ; 

 and there must be some means of replacing the oxygen removed 

 from the air by the processes of combustion and putrefaction, as 

 well as by the respiration of animals. 



Both these causes are united in the process of vegetable life. 



The facts which we have stated in the preceding pages prove 

 that the carbon of plants must be derived exclusively from the 

 atmosphere. Now, carbon exists in the atmosphere only in the 

 form of carbonic acid, and therefore in a state of combination 

 with oxygen. 



It has been already mentioned, that carbon and the elements of 

 water form the principal constituents of vegetables ; the quantity 

 of the substances which do not possess this composition being in 

 a very small proportion. Now, the relative quantity of oxygen 

 in the whole mass is less than in carbonic acid ; for the latter 

 contains two equivalents of oxygen, whilst one only is required 

 to unite with hydrogen in the proportion to form water. The 

 vegetable products containing oxygen in larger proportion than 

 this, are, comparatively, few in number ; indeed, in many the 

 hydrogen is in great excess. It is obvious, that when the hydro- 

 gen of water is assimilated by a plant, the oxygen in combination 

 with it must be liberated, and will afford a quantity of this ele- 

 ment sufficient for the wants of the plant. If this be the case, 

 the oxygen contained in the carbonic acid is quite unnecessary in 

 the process of vegetable nutrition, and it will consequently escape 

 into the atmosphere in a gaseous form. It is therefore certain, 

 that plants must possess the power of decomposing carbonic acid, 

 since they appropriate its carbon for their own use. The forma- 

 tion of their principal component substances must necessarily be 

 attended with the separation of the carbon of the carbonic acid 

 from the oxygen, which must be returned to the atmosphere, 

 whilst the carbon enters into combination with water or its ele- 



