THE AIR. 2J:7 



when, in addition to these facts, we remember that 

 the air is always more or less damp, it is easy to 

 understand that plants can derive from the air, alone, 

 the greater part of the substances which they require 

 (41, 150). 



639. Although the air contains so large a propor- 

 tion of oxygen, and although that substance is in a 

 free state, that is to say, not combined with any 

 element, but ready to combine with any substance 

 for which it has an affinity, yet it does not seem that 

 plants derive the oxygen which they contain directly 

 from the air. 



640. In the same way, there is no evidence to show 

 that they are able to absorb nitrogen from the air. 

 It might have been supposed that plants would obtain 

 the nitrogen which they require, directly from the 

 air, which contains nearly four-fifths of that gas ; but 

 there is very good reason to believe that this is not 

 th^ case, and that plants can only obtain nitrogen, 

 or assimilate it, as chemists say, by absorbing it in 

 combination with some other substance. 



641. What has just been said with regard to oxy- 

 gen and nitrogen, is equally applicable to carbon and 

 hydrogen : the former is a solid substance, and there- 

 fore, as one might rightly conclude, plants cannot 

 absorb it in the separate state ; when combined with 

 oxygen in the form of carbonic acid gas, and possibly 

 also when in the form of carburetted hydrogen (131), 

 it can be absorbed by plants/ Hydrogen has never 

 been found in the air, except in a state of combina- 



