802 THE ROLE OF NITROGEN, ETC., I., 



recognised that the living cell as a physiological entity owes its 

 formative material, its nourishment, growth, and even movement 

 to chemical changes; and further, that it contains some self- 

 regulating mechanism by which, amidst the continuous changes, 

 an almost constant chemical composition is maintained. 



The living protoplasm is constantly associated with protein, of 

 which the characteristic element is nitrogen; we therefore find 

 the latter in every living cell, and it is the main purpose of this 

 review to trace the nitrogen cycle in nature through some of its 

 many stages. 



The Source of the Nitrogen of Plants. 



The nitrogenous compounds which are found in every living 

 cell are of primary importance to organic life, and have their 

 origin in the atmospheric nitrogen, from which they are formed 

 by many complex changes. Not from the lightning flash alone, 

 but far more from the continuous and silent electric discharges 

 between clouds and from cloud to earth, comes the energy by 

 which the elements of water-vapour and air are united to form 

 ammonia, nitric and nitrous acids. This was the discovery made 

 by Priestley in 1774 when he recognised the oxides of nitrogen 

 in the air. In 1804, de Saussure detected ammonia in air, and 

 by very rough gasometric experiments showed that the free 

 nitrogen is not utilised by plants. A few years later, Chevreul 

 and Brandes found that ammonia was present in rain and river- 

 water; but it was Liebig who first called attention to the great 

 significance of ammonia in the rainfall on the growth of vegeta- 

 tion. He also detected nitric acid in rain water, but attached 

 very little importance to its nitrogen. Boussingault was the first 

 to make elaborate and exact quantitative estimations of ammonia 

 in rain water; and Barral of Paris, in 1851, made the first series 

 of quantitative nitric acid estimations in rain. Since then the 

 most important results are those of Gilbert and Lawes of 

 Rothamsted. 



The quantity of ammonia in the atmosphere, as the mean of 

 many results obtained by the best authorities in different 



