CHAP. XII. 
RESPIRATION. 
385 
instance, a plant of Mercurialis had its roots divided into two 
parcels, of which one was immersed in the neck of a bottle 
filled with a weak solution of acetate of lead, and the other 
parcel was plunged into the neck of a corresponding bottle 
filled with pure water. In a few days the pure water had 
become sensibly impregnated with acetate of lead. This, 
coupled with the well-known fact that plants, although they 
generate poisonous secretions, yet cannot absorb them by 
their roots without death, as, for instance, is the case with 
Atropa Belladonna, seems to show that the necessity of the 
rotation of crops is more dependent upon the soil being 
poisoned than upon its being exhausted. 
While oxygen and carbon are thus essential to vegetation 
when not administered in excess, almost all other gases are 
more or less deleterious. 
Although nitrogen is, as has already been shown, an im- 
portant and constant element in vegetation when dissolved or 
obtained by the decomposition of the atmosphere, yet in a 
pure gaseous state it seems incapable of affording any support 
to the developement of plants, as proved by Theodore 
de Saussure, who found that, five days after immersion in 
pure nitrogen, the buds of poplars and willows were in a state 
of decay. But he inclined to ascribe the apparent incapa- 
bility of leafy plants to absorb nitrogen to the artificial con- 
ditions under which the experiments were conducted. And 
this is probable, considering the nature of modern discoveries 
with respect to the action of nitrogen in vegetation. 
Pure hydrogen seems to act unequally upon vegetation. 
Saussure found that a plant of Lythrum Salicaria, after five 
weeks, had caused no alteration in a known volume of hy- 
drogen by which it was surrounded, and had not itself expe- 
perienced any apparent effect. Sir Humphry Davy, how- 
ever, states that some plants will grow in an atmosphere of 
hydrogen, while others quickly perish under such treatment. 
Drs. Turner and Christison found that so small a quantity 
as YoUo 0 sulphurous acid gas, a proportion so minute as 
to be imperceptible to the smell, was sufficient to destroy 
the life of leaves in forty-eight hours. The same observers 
state, in an excellent paper in Brewster's Journal for January, 
c c 
