Three forms are officinal: (1) Creta - chalk - the native, friable, and not pure carbonate; (2) Creta preparata - prepared chalk - the same substance well washed, or "elutriated," after being reduced to fine powder; and (3) Calcis carbonas prcecipitata - precipitated carbonate of lime.

1. Ordinary chalk is used only to produce carbonic acid gas in the making of carbonates, etc.

2. Prepared chalk occurs either in white powder or in small conical masses. The process of "elutriation" consists in treating the powder with a large quantity of water, allowing it to stand for a time, decanting from heavy particles, and allowing the milky liquid to gradually deposit -this form is used in mistura cretae) and pulvis cretae aromaticus.

3. Precipitated carbonate of lime is prepared by mixing a solution of carbonate of soda in excess, and at a boiling temperature, with solution of chloride of calcium. Carbonate of lime and chloride of sodium are formed, and the precipitate is washed until all the latter salt is removed. This preparation being crystalline and somewhat gritty, constitutes an ingredient of tooth powders, but is not otherwise recommended except in bismuth lozenges.