244 
rapidly forms, and carbonic acid is decomposed, stf 
as to furnish oxygen to the atmosphere in a quantity 
that appears to exceed considerably) diat which the 
plant, by its vegetation, is, in the same time, able to 
consume. If, indeed, in certain circumstances, the 
decomposition of carbonic acid by plants did not ex- 
ceed its formation, it is difficult to conceive how the 
composition of the air could be maintained unchang- 
ed in the experiments of M. de Saussure (305.) ; 
since the portion of time, in which the plants were 
exposed to the sun, was much less than that in which 
they were kept in the shade ; and, even in sunshine, 
they were equally employed in forming carbonic 
acid, although, on many occasions, no trace of it 
could be discovered. They must, therefore, in the 
shorter period in which they were exposed to the 
sun, have decomposed not only all the acid which 
they had previously formed in the shade, but all that, 
also, which their vegetation produced while this de- 
composition was going on." These considerations, 
therefore, raise our view of the extent and impor- 
tance of this decomposing power in plants ; but, 
giving to them all the weight they deserve, we think, 
from the various facts and circumstances already 
stated, that growing vegetables deprave the atmos- 
phere in a degree that greatly surpasses their power 
to ameliorate and improve it. 
530. But if the operation of vegetables in purify- 
ing our atmosphere be, even under the most favoura- 
ble circumstances, merely negative, and if, upon the 
whole, they must, like animals, be considered great- 
ly to deprave it, where, it may be asked, are we to 
