CARBONIC ACID A FOOD FOR PLANTS. 61 



tity of powdered marble be placed in any vessel and strong 

 vinegar, or any acid, is poured upon it, active effervescence 

 ensues and the carbonic acid is given off copiously. Chalk 

 is a common form of this combination of lime and carbonic 

 acid, the union of which forms carbonate of lime. One- 

 cubic inch of marble or chalk will yield 4 gallons or near- 

 ly half a cubic foot of this gas; and the burning of one bushel 

 of charcoal will produce 2,500 gallons. It is also produced 

 by fermentation. When cider is suffered to ferment; or 

 any other liquid which contains sugar; bubbles of carbonic 

 acid gas are evolved from it and rise through it and es- 

 cape at the surface. This is caused by the change of the 

 sugar into alcohol by which carbonic acid is formed. The 

 same result happens when a solution of malt or glucose 

 is fermented for the manufacture of beer : the foam which 

 appears upon the fresh beer being caused by the escape of 

 carbonic acid from the liquid during its confinement in the 

 barrel or bottle. The foaming of sparkling wines is due to 

 the same cause. 



It is also produced by the decomposition of solid sub- 

 stances which contain starch, or other vegetable matter. 

 The carbon of the starch, or cellular substance, is slowly 

 consumed by the low heat of the decomposition, and unites 

 with oxygen, giving off carbonic acid in the process; the 

 residue left after final decay being mostly all mineral 

 matter. 



Carbonic acid is the principal food of plants and con- 

 tributes largely to that portion of their substance, which 

 is derived from the atmosphere. The supply of this nec- 

 essary compound is derived both from the atmosphere, and 

 from the water, which are always present in the soil. 

 These entering into the substance of the plants, the for- 

 mer by the leaves and the latter by the roots, are taken in- 

 to the circulation in the sap and elaborated into the solid 

 cellular tissue, starch, sugar, and gum, which are com- 

 pounds of carbon oxygen and hydrogen; or carbon and 

 water; as the oxygen and hydrogen exist in these sub- 

 stances in precisely the proportions which go to form w r ater. 



