EAKLY THEORIES 3 



is burned. From these simple substances the plant elaborates 

 those compounds of carbon and nitrogen, such as starch, 

 sugar, fat, and the proteins, which the animal requires for 

 its food, and thereby reconverts into the original simpler 

 materials. Liebig's brilliant essay excited universal attention 

 and roused the interest of both the scientific and practical 

 men of all civilised countries in the subject, so that to 

 a very large extent we can date modern agricultural science 

 from this stimulating publication. Henceforward we may 

 take it that the source of the carbon of vegetation was no 

 longer regarded as doubtful; it came from the atmosphere, 

 and the humus of the soil practically contributed nothing 

 to it. 



The origin of the nitrogen was however by no means so 

 settled : De Saussure had concluded that plants were unable to 

 assimilate the free nitrogen of the atmosphere, but obtained it 

 from the nitrogenous compounds in the soil and from the small 

 amount of ammonia which he showed to be present in ordinary 

 air. Boussingault took out statistics of the nitrogen as well as 

 the carbon supplied in manures and recovered in the crops ; in 

 1838 he also published an account of experiments in which 

 plants were grown in pots and supplied with known amounts 

 of combined nitrogen, so as to ascertain if the growing plant did 

 assimilate atmospheric nitrogen. While the crop statistics 

 seemed to show in certain cases a considerable surplus of 

 nitrogen removed in the crops during a rotation over that 

 supplied in the manure, his direct experiments, made as 

 accurately as the chemistry of the time would permit, indi- 

 cated that plants drew nitrogen only from the soil or 

 manure. 



The arguments of De Saussure and of Boussingault were 

 adopted by Liebig in his first publication ; he considered the 

 source of the nitrogen of vegetation was ammonia derived from 

 the decay of the previous generation of plants or brought down 

 from the atmosphere by the rain. In his later editions Liebig 

 somewhat shifted from this point of view and began to minimise 



