85 
oxygen was afforded to it ; but that, when this acid 
gas was first mixed in certain proportions with the 
air, it constantly disappeared (310.), and the at- 
mosphere then contained a superabundant quantity 
of oxygen gas. The numerous and varied proofs of 
the decomposition of carbonic acid, afforded by the 
experiments of Priestley, Ingenhousz, Senebier, Da- 
vy, Woodhouse, and De Saussure, add great weight 
to the opinion for which we now contend, since they 
amount to demonstrative evidence, that carbonic 
acid is actually decomposed by the concurring ope- 
ration of the green parts of vegetables and of solar 
light. 
318. In what manner, then, does carbonic acid 
obtain admission into the vegetable, so as to be after- 
wards afforded by it in the form of oxygen gas ? It 
is well known, that this acid is readily attracted by 
water, and is almost universally contained in it. Wa- 
ter, it is likewise well known, is absorbed by the 
roots and leaves of plants ; and that it carries air 
with it into the vegetable body, is established by nu- 
merous observations. Dr Hales remarked air bubbles 
to rise, very abundantly, in hot weather, from vege- 
table sap collected in tubes, so as to make a froth an 
inch deep at the top ; and when, also, the sap was 
placed in an air pump, it afforded, as the receiver 
was exhausted, great plenty of air bubbles *. Mr 
Knight found the sap, whether extracted from the 
tree near to the ground, or at a distance from it, al- 
* Veg, Statics, vol. i. p. 125. 3d edit, 
F 2 
