Prep. By acting upon carbonate of lime, as chalk, marble, etc, with dilute sulphuric acid, and passing the gas through water under pressure.

Prop. & Comp. Carbonic acid (Co2) is a colourless gas, heavier than air, soluble in its own volume of water; the solubility much increased by pressure. The solution is acid in re-action, sparkling when exposed to air from the escape of the gas. Water containing this acid has the power of holding in solution carbonates of magnesia, lime, iron, etc.

Therapeutics. The gas, when existing in quantities above a very small amount in the air, acts as a narcotic poison, causing asphyxia; and directed in a stream upon a painful ulcerated surface, is stated to allay the pain. When taken in the stomach, aerated water diminishes irritability if present, and hence allays sickness; and carbonic acid is often given in the form of effervescing medicines made with an acid and bicarbonate of an alkali. The water may also be usefully employed in dissolving saline remedies, as phosphates, carbonates of potash, soda, and lithia, etc., when it is desirable to continue their use for a lengthened period.

Much of such water is now prepared in the Gasogene apparatus, of English and French construction.