82 
SI 7. IF we take a retrospective view of the cir- 
cumstances in which oxygen gas was produced in all 
the experiments which have been now detailed,- it 
will, we think, appear, that, conformably to the opi- 
nion of Senebier and Woodhouse, carbonic acid gas 
was always previously present. Thus, when Dr 
Priestley employed air that had been vitiated by re- 
spiration, combustion or putrefaction, which for the 
most part contains carbonic acid, the air was fre- 
quently much ameliorated and rendered capable of 
again supporting vegetable life ; but when he with- 
drew the oxygen gas of the atmosphere, by means 
which imparted to it no carbonic acid (247-X tne 
plants never failed to die. So, likewise, M. Scheele 
tvas never able to render vitiated air pure, even by- 
exposure to the sun, because he previously abstract- 
ed (247.) from it the carbonic acid which it contain- 
ed. In the air, depraved by respiration and combus- 
tion, which Ingenhonsz employed (29O. 1 .), carbonic 
acid seeras always to have previously existed. The 
experiments and observations of Senebier (298. 9.) 
-and Woodhouse (304.) altogether tend to prove, 
that oxygen gas is never afforded by the leaves of 
plants, either in water or in air, unless carbonic acid 
be present ; and that the quantity of oxygen supplied 
is in proportion to the carbonic acid that disappears. 
Lastly, in the experiments of De Saussure we have 
seen, that, when no carbonic acid previously existed: 
10 atmospheric air (305.), no additional quantity of 
