THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 445 
Looking, therefore, at the subject from the point of view of actual quantity merely, 
the ammonia in the atmosphere would appear very inadequate to yield Nitrogen in a 
degree at all corresponding to the yield of carbon by carbonic acid. It would appear 
too, from the observations hitherto recorded bearing upon the point, that the amount of 
Nitrogen existing in the atmosphere as nitric acid is very much less than that existing 
as ammonia. Hence, the inclusion, in the estimate of the combined Nitrogen in the 
atmosphere, of the amount existing as nitric acid would, in point of quantity, by no 
means materially affect the question. 
But it is worthy of remark, in reference to the question of the proportion of Nitrogen 
as ammonia to carbon as carbonic acid, that may be available to vegetation from 
atmospheric sources, that, although the actual amount of Nitrogen as ammonia in the 
atmosphere is very small compared with that of the carbon as carbonic acid, yet, a 
given amount of water would absorb very much more Nitrogen as ammonia, or dissolve 
very much more Nitrogen as carbonate of ammonia, than it would absorb of carbon 
in the form of carbonic acid under equal circumstances. In illustration, it may be 
mentioned that water at 60° F. (about 15°°5 C.) would at the normal pressure absorb. 
about 850 times as much Nitrogen in the form of ammonia as it would of carbon in 
the form of carbonic acid; and, under equal circumstances, very many times more 
Nitrogen as carbonate or even as bicarbonate of ammonia would be dissolved, than 
there would be of carbon as carbonic acid absorbed. There would appear to be, then, 
a compensating quality for the small actual amount of Nitrogen as ammonia in propor- 
tion to carbon as carbonic acid in the atmosphere, in the greater absorbability or solu- 
bility of the compounds in which Nitrogen exists than of the carbonic acid in which 
the carbon is presented. How far, however, the compensating quality here suggested 
may really influence the proportion of the Nitrogen to the carbon available from the 
atmosphere, in the combined form, under the actual conditions involved in vegetation, is 
a question the numerous and intricate bearings of which we do not profess here to 
enter upon. 
Before passing from this question of the direct absorption of Nitrogen in the com- 
bined form from the atmosphere by plants themselves, one or two further observations 
may yet be made which are suggested by the actual facts of agricultural production. 
It is undoubtedly the case that the Graminaceous crops depend very materially upon 
combined Nitrogen within the sotl, to determine the amount of their produce. They 
seem, however, to be comparatively independent of carbonic acid yielded by manure 
within the soil. The Leguminous crops, on the other hand, appear to be much less 
benefited by direct supplies of characteristically nitrogenous manures. It would hence 
seem that they are more able to avail themselves of Nitrogen supplied in some way by 
the atmosphere, possibly by the aid of their green parts. But it can hardly be toa 
greater mere eatent of surface above ground that the property which the Leguminous 
plants possess of acquiring a greater amount of Nitrogen than the Graminaceous ones, 
over a given area of land, and under otherwise equal circumstances, is to be attributed. 
