140 BIOLOGY OF PLANTS. 



stituents of mould ;" that is, he denies the existence of these 

 substances in the soil, and that they exert the slightest influ- 

 ence upon vegetation. But with this exception of Liebig and 

 Raspail, all chemists admit the existence of humic acid in 

 the soil. It should be observed further, that the humic acid 

 of soils is constant in its composition, while that formed by 

 artificial processes, varies in the proportion of its carbon. 



" Once for all," says Dr. Dana, when speaking of the hu- 

 mic acid of soils, " I consider ulmin, humus, geine, ulmic 

 and geic acid one identical substance, whether neutral or 

 acid, its constitution ever one and the same, subject to the 

 great law of organic chemistry, that proximate compounds 

 act as simple elements."* 



But whatever name we give to this substance, whether we 

 regard it as a definite organic compound in the soil, gen- 

 erally united to oxides, forming in fact a mass of geates, or 

 as composed of several acids, which are also combined with 

 similar bases, forming crenates, apocrenates and humates, or 

 whether it is differently constituted ; it has been generally 

 believed to be the source from whence plants derive most of 

 their carbon. 



There is one distinguished chemist, however, Liebig, who 

 has lately advocated the opinion, that the humus of soil does 

 not yield the " smallest quantity of carbon to plants," and 

 " that the only use of it is, to form a small quantity of carbonic 

 acid, a substance which exists in sufficient abundance in the 

 atmosphere, and from which alone, all the carbon is derived." 

 This theory, with others, will now be examined at length. 



I Theory oj Liebig. According to this theory, the only 

 source of the carbon of plants, is carbonic acid, which is 

 found either in the atmosphere, and absorbed by the leaves 

 of plants, or is dissolved in water, and enters by the roots. 

 In both cases, the acid is decomposed in the leaves, by the 



* For a full history of this substance, see Dana's Muck Manual, 

 p. 72 seq. 



