PHYSIOLOGY OF NUTRITION 103 



synthesis of organic compounds which, just before this 

 date, had been effected by Wohler, sounded the death- 

 knell of the " vital force " that had dominated the minds 

 of physiologists for over half a century. After 1840 

 we hear no more of this deus ex machina that was 

 dragged in to account for everything that could not be 

 explained by a reference to the known laws of physics 

 and chemistry. 



Another name came very prominently to the front 

 about 1840, viz. that of Liebig, who wrote a famous 

 treatise entitled Die organische Chemie in Hirer Anwendung 

 auf Agrikultur und Physiologie. Liebig was rather an 

 agricultural chemist than a botanist, and was therefore 

 primarily interested in the nature of the soil constituents 

 and the use the plant made of them. One of the greatest 

 services he rendered to botany was the refutation of the 

 " humus theory," so tenaciously held by physiologists 

 and agriculturists alike. This theory, as you doubtless 

 remember, postulates that the chief source of the supply 

 of carbon and other materials to the plant is the waste 

 from the animal and vegetable kingdoms. Liebig ex- 

 ploded this view by showing that humus was not absorbed 

 by roots and that, so far from reducing it in quantity, 

 as would naturally be the case if plants lived upon it, 

 the plant actually added to it. He further showed that 

 even if it were absorbed, the amount present in the soil 

 was quite insufficient to provide for the wants of vegeta- 

 tion. On the contrary, he insisted that the only source 

 of carbon available to the plant was the carbon dioxide 

 of the air. (Yet some years after Schleiden wrote " that 

 the leaves in their natural growth absorb carbonic acid 

 from the air was a pure invention " !) He also asserted 

 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 

 lifetime. Carbon dioxide, ammonia, and water are the 

 ultimate products of the chemical process of their putre- 



