THEORY OF LIEBIG. 141 



influence of solar light. This theory differs from others, only 

 in supposing, that all the carbon of plants is derived from the 

 carbonic acid of the atmosphere. All chemists and vegetable 

 physiologists fully agree, that a large portion of the carbon is 

 thus obtained. 



Arguments in support of this theory. Liebig represents 

 humic acid or humus, as the only substance in the soil, cajja- 

 ble of yielding carbon to plants ; and, as it requires 2500 parts 

 of water to dissolve it, alkalies or alkaline earths must com- 

 bine with it, to render it soluble and capable of entering the 

 roots. Now this acid is a definite compound, and he attempts 

 therefore, to determine the exact quantity which will combine 

 with the inorganic base, which enter into the composition of 

 vegetables. 



1. Suppose that the humate of lime, the most abundant 

 salt- of humic acid, is the source of the carbon. On the sup- 

 position that the lime remains fixed in the vegetable organs, 

 it is found by accurate calculation, that only Jg part of the 

 carbon, actually foupd in plants, could be introduced in this 

 way. 



2. If we calculate the quantity of metallic oxides in wheat 

 straw, and the known quantity of humic acid required to sat- 

 urate them, we shall find, that the proportion of carbon intro- 

 duced by this means, will be as 1 to 17, or about y^ part of 

 the carbon which the straw actually contains. 



3. Humate of lime, the most soluble of all the salts of hu- 

 mic acid, must first be dissolved in water, one part of which 

 requires 2500 of water for solution. Now if we calculate 

 the quantity of water which falls on a given surface, in one 

 season, and suppose all the water to enter the organs of plants, 

 the humate of lime which it would carry with it, would not 

 yield more than J^ of the quantity of carbon which is found 

 in the corn, grown on the same surface. But only a small 

 part of the water actually enters the roots of plants, and hence, 



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