gree in which one or other of these operations is 
found to prevail. 
,52?. If, then, from what has now been stated, it ap- 
pear, that living plants, at all times, and in all situa- 
tions, require the presence of oxygen gas, and, du- 
ring their growth, uniformly convert it into carbonic 
acid ; and if it also appear, that the same plants, on- 
ly at some times, and in some situations, decompose 
carbonic acid, and thereby afford oxygen gas to the 
atmosphere^ it evidently follows, that, as the con- 
sumption of oxygen is uniform and general, it must 
exceed, in extent, its p* oduction, which is only occa* 
sional and partial. Add to this, that the plants 
which, during sunshine, are employed In producing 
oxygen, are, at the same time, engaged in consum- 
ing it ; so that, even during that period, they may 
probably make no absolute addition of oxygen to the 
air ; and, at other periods, they must, in common 
with other plants, directly deprave it. When, how- 
ever, these two operations go on at the same time in 
sunshine, it is difficult to estimate the rate at which 
they respectively proceed, and, consequently, to de- 
termine the ratio which they bear to each other. 
528. To the consumption of oxygen gas by plants 
which grow in the open atmosphere, no other limits 
appear to be placed than those which are afforded 
by the size of the plant, and the extent and vigour 
of its vegetative power ; but the production of this 
gas by plants must depend on the supply of carbonic 
acid, and on the presence and efficiency of solar light. 
In consequence of the power which plants possess of 
simultaneously forming and decomposing (308.) car- 
