241 
they enter, but, likewise, the structure and properties 
of the instruments, by which these combinations are 
effected. It is, we believe, chiefly from inattention 
to these circumstances, that chemical hypotheses in 
physiology, though apparently consistent with che- 
mical principles, have seldom received the approba- 
tion of those, who, from their knowledge of the struc- 
ture and properties of living bodies, are, perhaps, 
best entitled to decide upon them; and, notwith- 
standing the confidence with which such hypotheses 
have been sometimes proposed, they have rarely ex- 
tended beyond the confines of the laboratory in 
which they were formed. 
526. Now that we have distinctly considered the 
effects which growing vegetables produce in the air, 
both when they are exposed to the sun, and when 
they are placed in the shade, it may, perhaps, be ex- 
pected that we should add a few observations on the 
general question of the purification of the atmosphere 
by vegetation, to which so much importance has 
been attached. It appears, then, that oxygen gas is 
essential to vegetation; and that plants, at all times, 
both in sunshine and in the shade, convert it into 
carbonic acid. In sunshine, however, and probably, 
also, in some degree, in the shade, they possess the 
power of reconverting carbonic acid into oxygen 
gas ; hence, therefore, the question, as to the abso- 
lute depravation or purification of the atmosphere by 
vegetation, jiuist be decided by the difference in de- 
