126 



LECTURE VIII. 



in combination in the form of water, or in the form of ammo- 

 nia and its compounds, or, as mentioned above, in complex 

 carbon compounds. 



Its use in the economy is very much the same as that of 

 carbon. 



Oxygen. Oxygen is taken up by plants either free, or in 

 combination in the form of water or of salts ; the free oxygen 

 absorbed is especially concerned in the processes of destruc- 

 tive metabolism, the combined oxygen in those of construc- 

 tive metabolism. 



It was pointed out in Lecture V. (p. 75), that certain 

 plants are capable of living, for a time at least, without taking 

 up free oxygen ; this point will be further considered in con- 

 nexion with the destructive metabolism of plants. 



Nitrogen. It has been conclusively proved by the re- 

 searches of Lawes, Gilbert, and Pugh, and by those of Bous- 

 singault, that plants are incapable of taking up free nitrogen : 

 they absorb it therefore only in combination (see p. 84). 



The annexed figure will give some idea of the method by which 

 Boussingault arrived at this result : a glass bell-jar A stands in a glass 



FIG. 20. 



dish C, which contains water strongly acidified with sulphuric acid, rest- 



