INTRODUCTION. XXlll 



advanced numerous arguments to prove that plants 

 derive the carbon which they contain from the de- 

 composition of that gas; yet this doctrine, although 

 admitted by many physiologists, was by no means 

 universally believed by chemists. M. Hassenfratz, 

 in particular, opposed these views, asserting that 

 plants did not derive their carbon from the decom- 

 position of carbonic acid existing in the air, bjit 

 absorbed it direct from the soil, in a state of suspen- 

 sion or solution; he gives the name of carbon to the 

 brown substance left on the evaporation of dung 

 water, and, in fact, to the various modifications of 

 decaying organic matter, subsequently described 

 under the general name of Humus. Few experi- 

 ments, indeed, were made to show that the explana- 

 tion of Priestley and Ingenhousz was improbable; 

 but 'it was conceived that plants must derive their 

 carbon from the soil, and many theories were formed 

 to explain the mode in which they might be supposed 

 to obtain it. These theories have been rigidly ex- 

 amined by Liebig, and the results of his investigation 

 have shown, that the old views put forth by Priestley 

 and Ingenhousz were in truth correct. 



It has long been known that plants consist of 

 Carbon, Oxygen, Hydrogen, and Nitrogen, and also 

 that they invariably contain a small quantity of inor- 



