71 
303. As in the preceding experiment, the plant 
was unable to decompose carbonic acid without the 
aid of light, so likewise he found that light, by it- 
self, was alike insufficient for this purpose ; for no 
alteration was effected in this acid gas, though ex- 
posed, for any length of time, to the action of the 
solar rays *. It follows from the foregoing series of 
facts, that neither the power of the plant alone, nor 
the agency of light alone, is able to decompose car- 
bonic acid, but that its decomposition is effected by 
the power of the plant, in concurrence with the direct 
agency of the solar rays. 
304. Dr Woodhouse exposed the leaves of nine 
different plants in separate vessels of atmospheric air, 
mixed with about ^ of carbonic acid, to the light 
of the sun for seven hours ; at the end of which 
time, he found that the acid gas had disappeared, 
and the atmospheric air was greatly increased in 
purity. When the air was previously vitiated by 
carbonic acid, formed by the putrefaction of a fun- 
gus, he found it to be restored to a great degree of 
purity by fresh leaves exposed to the sun ; but when 
similar leaves were kept in air through the night, 
carbonic acid was always produced, and the purity of 
the air thereby diminished. He maintains, as Sene- 
bier had before done, that oxygen gas is never pro- 
duced by plants, unless carbonic acid previously ex- 
ist in the air ; and he ascribes the production of oxy- 
gen gas, in Priestley's experiments on vitiated air, to 
the pre-existence of carbonic acid in it f* 
* Contributions, p. l6l. 
f Nicholson's Journal, 8vo, vol. ii. p. 1 53 ; & seq. 
