CHAP. X. 
RESPIRATION. 
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 shew 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. 
That nitrogen, per se, is incapable of affording any support 
to the development of plants, was proved by Theodore 
de Saussure, w^ho found that, five days after immersion in 
pure nitrogen, the buds of poplars and willows were in a state 
of decay. This is remarkable, considering how large a pro- 
portion of the air we breathe consists of nitrogen. It is, how- 
ever, to be remarked, that tliis is, in some measure, at variance 
with the most recent experiments of the same admirable ob- 
server, who finds that germinating seeds in all cases absorb 
nitrogen more or less. This, he says, happens sometimes in a 
very remarkable degree, but sometuiies so slightly that it might 
be attributed to errors in observation if it were not for the 
constancy of the results. This circumstance cannot be ascribed 
to simple porosity; for germinating seeds, after having re- 
mained several days in the air, or for a time sufficiently 
long to be saturated with nitrogen, still continued to absorb it. 
And, upon the whole, he seems inclined to ascribe the 
apparent incapability of leafy plants to absorb nitrogen to the 
artificial conditions under which the experiments were con- 
ducted. 
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 hydrogen by 
which it was surrounded, and had not itself experienced any 
apparent effect. Sir Humphrey Davy, however, states that 
Y 
