Carbonic acid, the aqueous solution of the gas, forms a series of exceedingly valuable salts called " carbonates." They are obtained by the direct action of the acid, or by the joint operation of the anhydride and water, upon metallic oxides or hydrates. In no case does the anhydride unite with a base without the intervention of water. Certain of the carbonates are also very readily produced by precipitating a soluble metallic salt with an alkaline carbonate. For the most part they are soluble in water and insoluble in alcohol; they are decomposed by the action of heat, and of water with the aid of heat.
The carbonates constitute an exceedingly important set of compounds, whether they be viewed from a geological, chemical, or purely industrial standpoint. The limestones form no inconsider able portion of the earth's crust, and the value of these under their several forms of chalk, mountain limestone, and marble, is too well known to be insisted upon. The carbonates of potash, soda, ammonia, and lead, are prepared on a large scale for various purposes. Some of the most valuable metallic ores are carbonates. The most important of these salts are the carbonates of baryta,
copper, iron, lead, lime, magnesia, potash, soda, and zinc.
All the carbonates, soluble and insoluble, are decomposed with effervescence by the strong acids. The gas which is given off is colourless, and of a somewhat pungent odour. If it be passed into milk of lime, or into a mixture of chloride of barium and caustic ammonia, a white precipitate is thrown down, though often not until the liquor has been heated. The same effects are produced by passing the gas into a solution of acetate of lead. These precipitates dissolve with effervescence in dilute nitric acid.
In analyzing a substance in order to determine the quantity of carbonic acid in combination, it is a common practice to ascertain the loss of weight which the substance undergoes when treated by other acids. It is necessary, however, when this method is adopted, to take care that vapour of water he not evolved along with the gas. The quantity of carbonic acid contained in lime and carbonate of lime is usually determined in this manner. J. L.