THEORY OF LIEBIG. 



143 



absorb the carbonic acid, assimilate the carbon, and yield 

 back the oxygen to the atmosphere, and they must always 

 have done the same. 



This remarkable property of plants has been demonstrated 

 in the most satisfactory manner by Priestley and Sennebier. 

 The power of decomposing the acid resides in the leaf, but 

 is exercised only when the leaf is exposed to the light. This 

 power is not dependent upon the connection of the leaf with 

 the stem, as leaves separated from the stalk, and exposed, in 

 an atmosphere of carbonic acid, to the solar light, readily ab- 

 sorb and decompose it ; but if the plant is immersed in an al- 

 kaline solution, which will prevent the carbon from being as- 

 similated, no oxygen will be emitted. Hence it appears, that 

 the life of plants is connected with that of animals, in a most 

 simple manner, and for a wise and sublime purpose. Plants 

 may live without animals, but animals must have organic 

 matter for their support. Plants purify the air, and furnish 

 an inexhaustible source of oxygen gas. 



The orfy questions now are, whether there is a sufficient 

 quantity of carbonic acid in the atmosphere to supply the 

 wants of plants ; and if so, whether it is available ? 



As to the quantity. We know the exact weight of the 

 whole atmosphere ; for every square inch, on the surface of the 

 earth, weighs 151bs., of which -jo^^-q part by weight is carbonic 

 acid. By this data, the quantity of carbon in the form of car- 

 bonic acid amounts to nearly 3000 billions of pounds ; a quan- 

 tity more than the weight of all the plants, and all the strata 

 of mineral and brown coal, which exist upon the earth. This 

 carbon is, therefore, more than adequate for all the purposes 

 for which it is required. The proportional quantity of car- 

 bon contained in sea-water,* is still greater. 



That this carbon is available to plants, appears from the 

 fact that the winds, moving at the rate of sixty miles per hour, 



* 10,000 volumes of sea-water contain 620 volumes of carbonic acid. 



