THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 447 



But another and potent reason for investigating the relation of plants to the free or 

 uncombinecl Nitrogen of the atmosphere is to be found in the fact, that the question 

 has, of late years, been submitted to an immense amount of research by numerous 

 experimenters, and from the results obtained very opposite conclusions have been 

 arrived at. Thus, M. Boussingault concludes that plants do not assimilate the free 

 or uncombined Nitrogen of the atmosphere. M. G. Ville maintains, on the contrary, 

 that the assimilation of free Nitrogen does take place, and further, that, under favour- 

 able circumstances, a considerable proportion of the Nitrogen of a plant may be derived 

 from this source. Others have experimented in connexion with the subject on a more 

 limited scale ; and various explanations have been offered of the discrepant results and 

 conclusions of M. Boussixgault and M. G. Ville. 



Before entering upon the discussion of our own experimental evidence in regard to 

 the question of the assimilation of free or uncombined Nitrogen by plants, it will be 

 desirable to pass in review the methods, results, and conclusions of M. Boussingault 

 and M. G. Ville, and also of some other experimenters, who seem to have been led to 

 take up the subject by a consideration of the contrary opinions arrived at by Boussingault 

 and Ville. 



Sectiox IV.— REVIEW OF THE RESEARCHES OF OTHERS, ON THE QUESTION OF THE 

 ASSIMILATION OF FREE NITROGEN BV PLANTS, AND ON SOME ALLIED POINTS. 



It has already been mentioned that, in 1837, Boussingault took up the question of 

 the sources of the Nitrogen of Plants where De Saussure had left it more than thirty 

 years before. De Saussure and his predecessors had sought to solve the question, among 

 others, whether plants assimilated the free or uncombined Nitrogen of the atmosphere, 

 by determining the changes undergone in the composition of limited volumes of air by 

 the vegetation of plants within them. Boussingault pointed out that the methods 

 which had been adopted were not sufficiently accurate for the determination of the point 

 in question. The general plan instituted by himself, and adopted with more or less 

 modification in most subsequent researches, was : — 



To set seeds or plants, the amount of Nitrogen in which was estimated by the analysis 



of carefully chosen similar specimens. 

 To employ soils and water containing either no combined Nitrogen, or only known 



quantities of it. 

 To allow the access, either of free air (protecting the plants from rain and dust), 

 of a current of air freed by washing from all combined Nitrogen, or of a fixed 

 and limited quantity of air, too small to be of any avail so far as its compounds of 

 Nitrogen were concerned. And finally — 

 To determine the amount of combined Nitrogen in the plants produced, and in the 

 soil, pot, &c, and, so, to provide the means of estimating the gain or loss of Nitrogen 

 during the course of the experiment. 

 wdccclxi. 3 Q 



