CHAP, ii.] of Plants. Liebig. 527 



Moreover, he was able to rest on long-accepted facts in just 

 those points which were the most important, and on these he 

 had only to throw the light of his chemical knowledge to 

 dispel the previous darkness. In accordance with his main 

 purpose, which was to apply organic chemistry and vegetable 

 physiology to the service of agriculture, Liebig directed the 

 severity of his criticism first of all against the humus-theory 

 constructed by chemists and agriculturists and thoughtlessly 

 adopted by various physiologists ; this was the first thing that 

 must be got rid of, if the question was to be answered, of what 

 substances does the food of plants consist, for the humus- 

 theory was at once incorrect, and the product of a want of 

 reflection which overlooked facts which lay before men's eyes. 

 Liebig showed that what was known as humus is not diminished 

 but constantly increased by vegetation, that the quantity in 

 existence would not suffice for any length of time for the 

 support of a vigorous vegetation, and that it is not taken up 

 by plants. This once established, and Liebig's calculations 

 left no doubt on the point, there remained one source only for 

 the carbon of the plant, namely, the carbon dioxide of the 

 atmosphere, with regard to which it was shown by a very 

 simple calculation resting on eudiometric results that its 

 quantity is sufficient to supply the vegetation of the whole 

 earth for countless generations. It is true that Liebig in his 

 zeal went much too far, when he found something contradictory 

 in the true respiration of plants, because it is connected with 

 the elimination of carbon dioxide, and simply denied its reality. 

 On the other hand the theoretical significance of the fact 

 established by de Saussure, that the elements of water are 

 assimilated at the same time as the carbon, was now for the 

 first time clearly explained. Liebig was better able to realise 

 the importance of this fact for the theory of nutrition than 

 de Saussure had been. But these weighty points were not the 

 ones which attracted most attention with the adherents and 

 opponents of Liebig ; the practical tendency of his book made 



