THE SOURCES OF THE ISHTEOGEN OF VEGETATION, ETC. 
471 
matter in them, and its relations to the inorganic substances, entirely precludes the possi- 
bility of oiu' imitating, by artificial means, a natural soil, so as to include all its condi- 
tions excepting a supply of combined Nitrogen. 
It is exident, therefore, that if all the conditions embraced in an ordinary soil were 
essential to vegetable growth, the solution of the question of the assimilation of free 
Nitrogen by plants would involve difiicnlties which our means of investigation in the 
present state of science could not overcome. But the experiments to which attention 
has been directed in the history of this subject, as well as others, the details of which 
we shall give further on, show that such is not the case. They show that many of the 
complicated conditions of an ordinary soil may be entirely dispensed with, so as to bring 
the examination of it within our means of investigation, and yet to retain all the condi- 
tions of healthy growth. 
In the experiments of the first year, 1857, two kinds of soil, or matrix, were used. 
One was prepared from an ordinary soil, so as more nearly to imitate the usual con- 
ditions of vegetable growth. The other was prepared from volcanic pumice, with the 
view to eliminate certain supposed sources of error which the prepared soil might intro- 
duce. It was found, however, in the experiments of 1857, that there was no necessity 
for this difference of matrix, and hence, in the experiments of 1858, only prepared soil 
was used. 
The soil selected for the preparation of the matrix was a somewhat heavy one (clayey), 
resting upon chalk, and interspersed wuth flints. The large stones were removed by 
picking and sifting ; and the clayey lumps Avere powdered to prevent them from baking 
into hard nodules during ignition. An attempt to ignite in ordinary clay crucibles was 
not successful, owing to the reduction of the peroxide of iron to the state of black oxide, 
and to the formation of sulphides from the reduction of the sulphates present, as indicated 
by the vapours of sulphurous acid emitted during the ignition, and by the evolution of 
sulphide of hydrogen on the addition of an acid to the mass after cooling. 
The combustion proceeded satisfactorily in a large cast-iron muffle, through which a 
constant cuiTent of air could pass. The ignition was continued until a portion of the 
soil assumed, on cooling, the red colour due to peroxide of iron, and exhibited no trace 
of coaly matter. The mass thus prepared was taken from the muffle, and thrown into 
a large vessel filled with distilled water. The rvater rvas rendered highly alkaline by 
the quantity of caustic lime present. The fluid was decanted, and fresh portions of 
water added several times during eight or ten days, until all the soluble matter was 
removed. The residue was then dried, and retained for final ignition before being used. 
The feiTuginous and aluminous character of this soil-matrix pointed to the danger there 
might be of its acting as a porous body, to promote the formation of nitrogenous com- 
pounds independently of vegetable growth, on the one hand, or to absorb and retain the 
ammonia given to the plant, or that which might be formed from the nitrogenous matter 
of the seed, on the other. 
To ascertain the value of any influence exerted by the soil independently of the plant, 
MDCCCLXI. 3 T 
