THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 469 
We shall dwell more fully upon the conditions involved in the experiments than 
upon the numerical results which they have afforded, since the value of these results is 
so wholly dependent on those conditions, that, if the latter are properly arranged and 
thoroughly considered, any conclusion with regard to the former will be sufficiently 
apparent from the numerical results themselves. 
In studying the conditions, we shall be obliged to touch upon several collateral 
points, embracing some questions not necessarily involved in the investigation, and 
which, therefore, we have not attempted to treat with that fulness which, as distinct 
questions in vegetable Physiology, they merit. Yet, we think, it will appear that, in 
the degree in which we have followed them, their discussion is essential to complete 
the consideration of the main question of the investigation, and that, in relation to it, 
they possess an interest quite commensurate with the attention we have devoted to them. 
These questions are embraced in the following :— 
1. The preparation of the soil or matrix for the reception of the plant, and of the 
nutriment to be supplied to it. 
2. The preparation of the nutriment to be supplied to the plant,—embracing that of 
mineral constituents (as in the ash), of certain solutions, and of water. 
3. The conditions of the atmosphere to be supplied to the plant, together with the 
means of securing them,—involving a consideration of the circumstances affecting the 
composition of the atmosphere, and of the apparatus used to supply it. 
4. The changes undergone by nitrogenous organic matter during its decomposition, 
affecting the quantity of combined Nitrogen present, in circumstances more or less 
analogous to those in which the plants were grown in our experiments upon the assi- 
milation of Nitrogen. 
5. The action of agents, as ozone, and the influence of other circumstances which may 
affect the quantity of combined Nitrogen present in connexion with the plant, and yet 
independent of the direct action of the vital (growing) process. 
In considering these five questions, two important series of conditions must be 
fulfilled :— 
-1. Those that relate to the growth of the plant,—which must be so arranged as to 
include all that is necessary for healthy and vigorous growth, excepting only, in some 
instances, such conditions as may depend upon the presence of a supply of combined 
Nitrogen. 
2. Those that relate to our means of measuring the quantity of combined Nitrogen 
present at different periods of growth,—it being essential that we should be able, with 
the means of investigation afforded in the present state of science, to ascertain the 
quantity of combined Nitrogen present with the plant at different periods of its growth, 
with sufficient exactness to detect any changes that may take place, so as to enable us to 
refer them to their proper source. ; 8 
If we succeed in fulfilling all these conditions, we shall have at our command alt 
the data requisite for the solution of the question whether plants assimilate free 
