On the Sources-of the Nitrogen of Vegetation. 523 
tions of this-real- or- apparent loss of the nitrogen supplied by 
manure are enumerated. 
The question arises—what are the sources of all the nitrogen of 
our crops beyond that which is directly supplied to the soil by arti- 
ficial means? The following actual or possible sonrces may be 
enumerated :—the nitrogen in certain constituent minerals of the 
soil; the combined nitrogen annually coming down in the direct 
aqueous depositions from the atmosphere; the accumulation of 
combined nitrogen from the atmosphere by the soil in other ways ; 
the formation of ammonia in the soil from free nitrogen and nascent 
hydrogen; the formation of nitric acid from free nitrogen; the 
direct absorption of combined nitrogen from the atmosphere by 
plants themselves ; the assimilation of free nitrogen by plants. 
A consideration of these several sources of the nitrogen of the 
vegetation which covers the earth’s surface showed that those of 
them which have as yet been quantitatively estimated are inadequate 
to account for the amount of nitrogen obtained in the annual pro- 
duce of a given area of land beyond that which may be attributed 
to supplies by previous manurmg. Those, on the other hand, which 
have not yet been even approximately estimated as to quantity 
—if indeed fully established qualitatively—offer many practical 
difficulties in the way of such an investigation as would afford results 
applicable in any such estimates as are here supposed. It appeared 
important, therefore, to endeavour to settle the question whether or 
not that vast storehouse of nitrogen, the atmosphere, affords to grow- 
ing plants any measurable amount of its free nitrogen. Moreover, 
this question had of late years been submitted to very extended and 
laborious experimental researches by M. Boussingault, and M. Ville, 
and also to more limited investigation by MM. Mine, Roy, Cloez, 
De Luca, Harting, Petzholdt and others, from the results of which 
diametrically opposite conclusions had been arrived at. Before enter- 
ing on the discussion of their own experimental evidence, the Authors 
give a review of these results and inferences; more especially those 
of M. Boussingault who questions, and those of M. Georges Ville 
who affirms the assimilation of free nitrogen in the process of vege- 
tation. 
The general method of experiment instituted by Boussingault, 
which has been followed, with more or less modification, in most 
subsequent researches, and by the Authors in the present inquiry, 
was—to set seeds or young 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 (the plants being protected from rain and dust)— 
of a current of air freed by washing from all combined nitrogen—or 
of a limited quantity of air, too small to be of any avail so far as any 
compounds of nitrogen contained in it 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. 
