232 
in support of this supposed decomposition of water, 
and consequent production of oxygen, that we pro- 
pose now to examine. 
514. It has been said, that, when the leaves of 
plants are exposed to light, in contact with pure wa- 
ter only, they afford oxygen gas, -a proof that their 
vessels are able, when assisted by the action of light, 
to decompose a portion of the water which they ab- 
sorb *. This oxygen, however, may proceed from 
so many other sources, that the quantity afforded 
cannot be received as proof that it is derived from the 
decomposition of water ; and the small portion that 
is yielded by leaves, when thus placed in water, may, 
\vith greater probability, be ascribed to the air ac- 
tually existing in the substance of the leaf before its 
immersion. 
515. But an experiment of Mr Davy seems to af- 
ford the best evidence that has been adduced on this 
subject. He filled a glass cylinder, of the capacity 
of ten cubic inches, with mercury ; and then convey- 
ed into it two small vine leaves, which were passed 
through the mercury, so as to detach all atmospheric 
air from them. The mercurial apparatus was now 
inserted in a vessel of cold water ; and aqueous gas, 
furnished from another vessel containing water, 
which had been long in ebullition, was passed through 
a long tube into the cylinder, where it was conden- 
sed by the cold mercury. In this manner, the cylin- 
der was filled with water that held no air in solution, 
and, in this state, it was exposed to light. In a very 
* Murray's Syst, Chenu vol. iv. p. 
