20 3 
the external elements. The constituent parts of 
the air are subservient to this process ; but, as it 
might be expected, they act differently under dif- 
ferent circumstances. 
When a growing plant, the roots of which are 
supplied with proper nourishment, is exposed in 
the presence of solar light to a given quantity of 
atmospherical air, containing its due proportion 
of carbonic acid, the carbonic acid after a certain 
time is destroyed, and a certain quantity of oxygene 
is found in its place. If new quantities of carbonic 
acid gas be supplied, the same result occurs ; so 
that carbon is added to plants from the air by the 
process of vegetation in sunshine ; and oxygene is 
added to the atmosphere. 
This circumstance is proved by a number of 
experiments made by Drs. Priestley, Ingenhousz, 
and Woodhouse, and M. T. de Saussure ; many 
of which I have repeated with similar results. 
The absorption of carbonic acid gas, and the pro- 
duction of oxygene are performed by the leaf; 
and leaves recently separated from the tree effect 
the change, when confined in portions of air con- 
taining carbonic acid ; and absorb carbonic acid and 
produce oxygene, even when immersed in water 
holding carbonic acid in solution. 
The carbonic acid is probably absorbed by the 
fluids in the cells of the green or parenchymatous 
part of the leaf; and it is from this part that oxy- 
gene gas is produced during the presence of light. 
M. Senebier found that the leaf, from which the 
epidermis was stripped off, continued to produce 
oxygene when placed in water, containing carbonic 
