_Carbonic acid_ is formed and given out during the process of fermentation, putrefaction, respiration, &c. and makes 28 parts out of 100 of atmospheric air. It is composed (according to Davy) of oxygen and carbon, in the proportion of 34 of the former to 13 of the latter. It combines freely with many different bodies; animals and vegetables are almost entirely composed of it; for the _coal_ which they give, on combustion, is but _carbon_ united to a little oxygen, &c.--Priestly was the first to discover, that plants _absorbed carbonic acid_; and Ingenhouse, Sennebier, and De Sausure have proved, that _it_ is their _principal aliment_. Indeed the great consumption made of it, cannot be explained by any natural process, excepting that of vegetation. On this head, we cannot do better than digest the experiments of these chymists into a few distinct propositions:[2] [2] Recherches chemiques sur la vegetation, chap. ii. 1. In pure carbonic acid gas, seeds will swell, but not germinate. 2. United with water, this gas hastens vegetation. 3. Air containing more than one twelfth part of its volume of carbonic acid, is most favourable to vegetation. 4. Turf, or other carbonaceous earth, which contains much carbonic acid, is unfavourable to vegetation until it has been exposed to the action of atmospheric air, or of lime, &c. 5. If slacked lime be applied to a plant, its growth will be impaired, until the lime shall have recovered the carbonic acid it lost by calcination. 6. Plants kept in an artificial atmosphere, and charged with carbonic acid, yield, on combustion, more of that acid than plants of the same kind and weight growing in atmospheric air. 7. When plants are exposed to air and sunshine, the carbonic acid of the atmosphere is consumed, and a portion of oxygen left in its place. If new supplies of carbonic acid be given to the air, the same result follows; whence it has been concluded, that air furnishes carbonic acid to the plant, and the plant furnishes oxygen to the air.--This double function of absorption and respiration, is performed by the green leaves of plants.[3] 8. Carbon is to vegetation, what oxygen is to animal life; it gives support by purifying the liquids, and rendering the solids more compact. [3] This was a discovery of Sennebier. 4th. Of _light_, _heat_, and _electricity_, and their agency in vegetation: