18 THE SCIENTIFIC PRINCIPLES 



the farm itself, together with that due to the refuse 

 of the crops, must always be less than was contained 

 in the crops grown. He concluded that, although 

 the quantity returned to the land as manure was 

 important, a main source of the nitrogen assimilated 

 over a given area was the ammonia brought down 

 from the atmosphere in rain. 



There can be no doubt that, owing to the limited 

 and defective experimental evidence then at command 

 on the point, Liebig at that time (as he did afterwards) 

 greatly over-estimated the amount of combined ni- 

 trogen available to vegetation from that source. 



In Boussingault's • reclamation ' already referred 

 to, he gave much more prominence to the importance 

 of the nitrogen of manures. In Liebig's next edition 

 (in 1843) he combated the notion of the relative 

 importance of the nitrogen of manures. He main- 

 tained, in opposition to the view put forward in his 

 former edition, that the atmosphere afforded a 

 sufficient supply of combined nitrogen not only for 

 natural vegetation, but for cultivated plants ; that 

 this supply was sufficient for the cereals as well as 

 for leguminous plants ; that it was not necessary to 

 supply nitrogen for the former ; and he insisted very 

 much more strongly than formerly, on the relative 

 importance of the supply of the incombustible, or, 

 as he designated them, the ' inorganic = or ' mineral/ 

 constituents. 



Many determinations of the amount of combined 

 nitrogen brought down in rain have been made, in 

 different countries, since the date here referred to ; 

 and in a subsequent lecture it will be shown that 

 the amount so available to the vegetation of a given 

 area is very much less than was assumed by Liebig, 

 or has generally been supposed. 



