254 SCIENCE PROGRESS. 



What could be more natural in seeking for the explana- 

 tion of the supply of nitrogen, than to turn to the ocean of 

 free nitrogen in the atmosphere as the source of this 

 element ? 



Recourse to rigidly controlled experiments in the hands 

 of Boussingault (5) and of Lawes and Gilbert put beyond 

 all doubt, however, that under the conditions devised for 

 these experiments in closed apparatus, the typical agricultural 

 plants such as beans, peas, wheat, barley, potatoes, etc., 

 assuredly do not "fix' or directly make use of the free 

 nitrogen of the atmosphere, but must have their nitrogen 

 presented in some combined form, and especially as nitrates ; 

 whereas they do manufacture carbon compounds out of the 

 carbon-dioxide and water. 



It is extremely instructive and interesting to examine 

 the controversies which raged around these questions, and 

 to see how sound was the basis of pure experiment on which 

 their settlement was made. 



The only possible conclusion as regards the nitrogen 

 supply seemed to be that it depends on three principal 

 sources : (1) Mineral results of decomposition of rocks, 

 ejecta from volcanoes, subterranean springs, etc. ; (2) the 

 products of decay of animal and vegetable bodies, and (3) 

 the ammonia and nitric acid brought down in rain, and 

 principally furnished by the results of electric discharges in 

 the air. 



In fact, the green plant seemed to depend for its nitrogen 

 entirely on nitrates, or compounds convertible into nitrates, 

 absorbed by its roots and carried in the water supply to the 

 seat of activity where the higher nitrogenous compounds are 

 synthesised, and although we now know that this statement 

 is too wide, it serves as the basis of the general argument. 



Meanwhile and as time went on, a number of discoveries 

 and generalisations in various directions in botany were 

 having their bearing on the question. 



Parasitic and saprophytic plants were, of course, dependent 

 on green plants for their supplies of organic materials, and 

 the former especially were dependent on other organisms for 

 their nitrogenous compounds, just as animals are. 



