Origin. - Obtained from sodium sulphate and sodium chloride, but chiefly by a complicated process, known as Leblanc's, from sodium sulphate, which is mixed with chalk and coal, the mixture ignited, and the resultant mass exhausted with water and concentrated, the carbonate separating from the hot liquid being purified.

Description and Properties. - Soluble in 2.9 parts of water at 25o C. (77o F.), in 0.09 part at 38o C. (100.4 F.), in 1.8 parts of boiling water, and in 8 parts of glycerin; insoluble in alcohol and ether. The aqueous solution gives an alkaline reaction with litmus-paper, and effervesces strongly with acids. The drug should be kept in well-closed vessels.

Dose. - 2-15 grains (0.0125-1.0 Gm.) [4 grains (0.25 Gm.), U. S. P.].

The sodii carbon as contained 10 molecules of water of crystallization or 63 per cent.; part of this was lost on exposure to air, so that the salt was of uncertain strength. The sodii carbonas exsiccatus contained about 26 per cent. of water, and probably corresponded to the formula Na2CO3+ 2H1O. This salt was somewhat hygroscopic. The monohydrated salt does not effloresce at ordinary temperatures, nor does it absorb much moisture. It is, therefore, more uniform in composition than either of the others.