231 
In as far, however, as carbonic acid is decomposed 
by solar light, and thereby yields oxygen gas, in so 
far may it be considered useful to vegetation ; but it 
is not, under any circumstances, necessary to that 
process, if oxygen gas be duly supplied from any 
other source. 
513. BESIDES furnishing oxygen gas, by the de- 
composition of carbonic acid, many writers have sup- 
posed that the leaves of plants afforded it, also, by 
the decomposition of water. This opinion, how- 
ever, seems rather to have been inferred from rea- 
soning, than proved by experiment. Vegetables, it is 
said, augment their bulk, when furnished only with 
water and air ; and since oxygen and hydrogen (the 
constituents of water) are likewise two of the most 
abundant ingredients in plants, it is inferred that wa- 
ter suffers decomposition, and is thereby enabled to 
contribute to the nutrition of the plant. Because, 
also, the leaves of plants contain inflammable matter, 
and, in certain circumstances, emit oxygen gas, IVL 
Berthollet was, led to suppose, that, when exposed to 
the sun, they possessed the power of decomposing 
water, as well as carbonic acid. In this manner, he 
believed that the oxygen was derived from two 
sources ; while the hydrogen of the water and the 
carbon of the acid contributed to form the resinous 
or inflammable matter *. It is the evidence adduced 
* Thomson's Syst* Cheni. vol.. v. p. 36'2. 4th edit. 
