16 THE SCIENTIFIC PRINCIPLES 



treatment of his subject he not only called to his aid 

 the previously existing knowledge directly bearing 

 upon it, but he also turned to good account the more 

 recent triumphs of organic chemistry, many of which 

 had been won in his own laboratory. Further, a 

 marked feature of his expositions was the adoption of 

 the quantitative method of illustration, and argument. 

 But, notwithstanding the evidence afforded by the 

 direct experiments of De Saussure and his prede- 

 cessors, Vegetable Physiologists, and some others, con- 

 tinued to hold the view that the humus of the soil 

 was the source of the carbon of vegetation. Liebig 

 gave full weight to the evidence of the experiments 

 of De Saussure and others; he illustrated the pos- 

 sible or probable transformations within the plant 

 by facts already established in organic chemistry ; 

 and he demonstrated the impossibility of the humus 

 of the soil supplying the amount of carbon assimi- 

 lated over a given area. He pointed out, that 

 humus itself was the product of previous vegetable 

 growth; that it could not therefore be an original 

 source of the carbon ; and that, from the degree of 

 its insolubility, either in pure water or in water con- 

 taining alkaline or earthy bases, only a small portion 

 of the carbon assimilated by plants could be derived 

 from the amount of humus that could possibly enter 

 the plant in solution. He maintained that, so far as 

 humus was beneficial to vegetation at all, it was 

 only by its oxidation, and a consequent supply of 

 carbonic acid within the soil ; a source which he 

 considered only of importance in the early stages 

 of the life of a plant, and before it had developed, 

 and exposed to the atmosphere, a sufficient amount 

 of green surface to render it independent of soil- 

 supplies of carbonic acid. 



