528 Theory of the Nutrition [Book hi. 



the discussion, to which it gave rise especially among chemists 

 and agriculturists, turn rather on the question of the source 

 of the nitrogen in the substance of plants. The humus-theory 

 had made the nitrogen like the carbon enter the plant in the 

 form of organic compounds. De Saussure in his great work of 

 1804 had named ammonia as a compound of nitrogen which 

 might be taken into consideration with others, but he arrived 

 at no definite conclusion. Liebig, from different points of 

 view and in reliance on his own investigations into the nature 

 of nitrogen and its compounds, arrived at the result, that 

 ammonia must ultimately be the sole source of the nitrogen in 

 the plant, and that the ammonia in the atmosphere and in the 

 soil is quite sufficient to supply vegetation with the requisite 

 amount of nitrogen just as the carbon dioxide of the atmosphere 

 is the sole source of the carbon of the plant ; and so he con- 

 cluded that 'carbon dioxide, ammonia, and water contain in 

 their elements the requisites for the production of all the 

 substances that are in animals and plants during their life-time. 

 Carbon dioxide, ammonia, and water are the ultimate products 

 of the chemical process of their putrefaction and decay.' 



Liebig was less happy, at least as regards his mode of treat- 

 ing the subject, in his remarks on the necessity and specific 

 importance of the constituents of the ash to the nutrition of 

 plants. Instead of insisting on an experimental answer to the 

 question, what constituents of the ash are absolutely indispens- 

 able to the health of one or all plants, he lost himself in 

 ingenious chemical theories, intended to show the operation of 

 inorganic bases in fixing vegetable acids, the extent to which 

 different bases can replace each other, and similar matters. 



It is not requisite for our purpose to follow Liebig in his 

 applications of his theoretical remarks to agriculture, still less 

 to occupy ourselves with the sensation and the discussions 

 which his work excited among practical and theoretical 

 farmers and agricultural chemists. The scientific value of 

 Liebig's considerations on the nutrition of plants stood out in 



