
AS A SOURCE OF THE NITROGEN FOUND IN PLANTS. 597 
or to deoxidise them. The earliest teachers of this doctrine, CavenpisH, Wart, 
Mevusnier, and Lavoisier, supposed this deoxidising power to be chiefly expended 
upon water. Ata later period, when the fact that carbonic acid is an oxide of 
carbon was discovered, and PriesTLEy’s experiments on the conversion of fixed 
air into free oxygen, by the green leaves of plants in the presence of sunshine, 
were recalled, the deoxidising powers of a plant were supposed to be mainly 
expended on carbonic acid. At present, we should decline to say whether this 
acid or water was most the subject of deoxidation in plants; and we should add 
to those oxides, sulphuric acid, as constantly undergoing separation into its ele- 
ments. To such a conclusion we are driven by the fact, that whilst unoxidised 
sulphur is found in many of the constituents of plants, sulphates are the only 
compounds of sulphur which are found entering them. 
Whatever else, however, is doubtful, this is certain, and is acknowledged by 
chemists of every school, viz., that a plant is like a blast-furnace, which the sun 
kindles every day into full action; and that no oxide can pass through such an 
apparatus, without risking the loss of all its oxygen. With what consistency, 
then, can it be contended, that water, carbonic acid, and sulphuric acid, cannot 
pass through a plant in the presence of sunshine, without being deprived in whole 
or in part of their oxygen, but that the much more easily deoxidised nitric acid, in 
the same circumstances, will not suffer deoxidation? It might as well be affirmed 
that a blast-furnace may be competent to reduce the refractory oxide of iron, and 
yet be incompetent to reduce the easily reducible oxide of lead. 
No one I think will deny, that out of a plant, sulphates are deprived of oxygen 
with much more difficulty than nitrates are; if, however, the deoxidising force at 
work within a plant can deprive sulphates of their oxygen, @ fortior? it can deprive 
nitrates of their oxygen, and we must concede their deoxidability and deoxida- 
tion. 
But further, the alkaline nitrates which are the medium of the introduction 
of nitric acid into plants, will certainly within them separate more or less com- 
pletely into acid and alkali, and let the former become free. Those who contend 
that nitrate of soda profits plants only in so far as it contains soda, imply by this 
statement that the nitric acid is set free from the soda, and in some way disposed of. 
All chemists, moreover, will acknowledge that the large amount of fixed alkaline 
bases found present in every plant in union with organic acids, compels us, what- 
ever theory we hold, to look upon these acids developed within the plant, as having 
taken the place of the inorganic or mineral acids which accompanied the bases into 
its structure. Nitric acid must therefore be often set free within vegetable organ- 
isms; and when set free, must more rapidly than any uncombined inorganic oxide 
which is present in plants, suffer instant deoxidation. This proposition, I think, 
needs no proof. Uncombined carbonic or sulphuric acid, cannot be deoxidised by 
any known artificial process, so as to separate its oxygen as free gas. Water can 
be made to yield free oxygen only by a powerful voltaic eurrent,—by an intense 
