7$ 
addition of oxygen gas was made to the atmosphere 
in which the vegetables grew, but both its volume 
and composition remained permanently the same. 
Even if decomposition of the vegetables had taken 
place, we do not know that they could have directly af- 
forded pure oxygen gas ; but we do know that this gas 
is furnished by them in union with carbon ; so that the 
existence of an additional quantity of oxygen, in such 
circumstances, would have afforded equal evidence of 
the decomposition of carbonic acid, by the agency of 
solar light. 
312. But not only by the direct rays of the sun, 
but without the intervention of a strong light, it is 
probable, says M. de Saussure, that plants, growing 
in atmospheric air, decompose a part of the carbonic 
acid, which they themselves form out of the sur- 
rounding oxygen gas, although the fact be not sus- 
ceptible of direct proof. Marsh plants, as the poly- 
goiiuni persicaria and the ly thrum salicana, yielded 
oxygen gas by a weak and diffused light, when con- 
fined in an atmosphere of nitrogen * : and different 
species of cpilobium vegetated a long time, and grew 
as well in pure nitrogen gas, as in common air, 
though exposed only to a weak light, or protected 
from the direct action of the sun. The nitrogenous 
atmosphere, at the end of two months, was increased 
in bulk, and contained -^ of oxygen gas. When 
similar plants were confined in pure nitrogen gas, and 
kept in perfect darkness, though they were renewed 
.JLecherches, p. 54. 
