69 
ced under water and exposed to the sun, afford pure 
air, when the temperature was many degrees below 
zero ; and many evergreens, under similar circum- 
stances, are always able to afford it. Tn every case, 
he believes the oxygen to be derived from the de- 
composition of carbonic acid, which he supposes to be 
effected in the leaves by the affinity of light for oxy- 
gen ; and the oxygen produced is, he adds, always 
proportional to the quantity of carbonic acid that is 
present. He denies that oxygen is ever derived 
from the decomposition of water ; for in boiled water, 
which contains no carbonic acid, plants do not afford 
oxygen, while in waters impregnated with this acid 
gas, they yield it most abundantly. Neither is the 
oxygen furnished solely by the leaves, independently 
of light, since it is not obtained from them when the 
apparatus is placed in the shade *. From these facts 
we collect, that the presence of carbonic acid is ne- 
cessary to the production of oxygen, which can be 
furnished only by the decomposition of this acid gas 
through the joint agency of the plant and of solar 
light. 
300. In 1779, Mr Davy made some experiments 
with the view of discovering the origin of the oxy- 
gen gas which vegetables, in certain circumstances, 
seem capable of affording. He introduced a small 
plant of arenaria tenuifcHa, growing in a pot of 
moist earth, into a jar containing 14 cubic inches of 
carbonic acid. The jar was inverted in mercury, 
and exposed to the solar rays, for four successive 
* I'll vs. \V. t. id. p, 195. et 
