CHAP, ii.] the Nutrition of Plants. 449 



plants to physico-mechanical principles ; de Saussure and others 

 showed that the heat of plants is a product of respiration, and 

 by 1840 the earlier theory of a vital force might be looked 

 upon as antiquated and obsolete. It remained to restore to 

 their rights the observations of Ingen-Houss and de Saussure, 

 which under the influence of that theory and of the notions 

 respecting the humus had been so utterly misconstrued. 

 Liebig set aside the humus-theory in 1840, and referred the 

 carbon of plants entirely to the carbon dioxide of the atmosphere, 

 and their nitrogenous contents to ammonia and its derivatives ; 

 he claimed the components of the ash as essential factors in 

 the nutrition, and taking his stand on the general laws of 

 chemistry endeavoured to obtain chiefly by the method of 

 deduction an insight into the chemical processes of assimilation 

 and metabolism. The whole theoretical value of the facts 

 discovered by Ingen-Houss, Senebier and de Saussure was 

 first made apparent by the connection which Liebig succeeded 

 in establishing between the phenomena of nutrition. The 

 doctrine of nutrition burst suddenly into new life ; firm 

 ground was gained, and the botanist, no longer distracted by 

 the difficulties raised by the vital force but resting on physical 

 and chemical principles, might now resume the task of in- 

 vestigation. Oxygen-respiration denied by Liebig was first 

 of all re-established by von Mohl and others. Liebig's views 

 on the source of nitrogen in plants and on the importance of 

 the ash-constituents rested chiefly on general considerations 

 and observations and on calculation, and had now to be tested 

 by systematic investigation and especially by experiments on 

 vegetation in individual plants. And here the place of honour 

 must be assigned to Boussingault, who pursued the path of 

 pure induction as contrasted with Liebig's deductive mode of 

 proceeding, gradually improved the methods for experimenting 

 on vegetation, and soon succeeded in so producing plants in a 

 purely mineral soil free from all humus, that he fin, illy settled 

 the question of the derivation of the carbon from the atmosphere 



