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MEANS FOR DETERMINING THE PURITY OF DRUGS, ETC 
water. It is totally volatile by the temperature of a water bath. 
Exposed to the air or kept insecurely, it decomposes readily, los- 
ing ammonia and carbonic acid, becoming opaque, lighter, and 
finally, a white crumbly mass of bicarbonate. This change occurs 
so easily, that when imported in stone-ware vessels not well 
glazed, it has become much injured. It is generally sufficiently 
pure for medicinal use, containing only traces of tarry matter, which 
gives a dark color to its solution in acids, and leaves a brownish resi- 
due when the salt is evaporated on a water bath. It becomes less 
fit for medicinal use as it undergoes decomposition by exposure. 
No portion of the salt should be used for medicinal purposes except 
it be clear and translucent, all that is white and opaque should 
be rejected, or used for those purposes only (saturation of acid, &c.) 
in which an excess of carbonic acid is not important. 
Carbonate of Lime. — This is officinal under three forms of 
different purity : creta preparata, testa preparata, and calcis car- 
bonas prcecipitatus , the latter introduced into the Pharmacopoeia 
of 1850. 
Prepared chalk and shell are in dull white friable lumps, inodor- 
ous and insoluble in water. Acted on by hydrochloric acid, di- 
luted with four to six parts of water, it dissolves with effervescence, 
forming a copious froth which does not readily subside, and the 
liquid contains dirty gray flocculi. The filtered liquid, much di- 
luted, produces a slight precipitate with caustic alkali, and a co- 
pious white precipitate with oxalate of ammonia. The prepared 
chalk is rarely adulterated, but if not well prepared may be grit- 
ty or dirty ; when mixed with water and allowed to stand a few 
moments, on decantation no harsh residue should have subsided ; 
the solution in hydrochloric acid should not have in it so much 
flocculi as totally to obscure its transparency. On account of the 
foliated texture of oyster shell, it is not readily reduced to tine 
powder, hence chalk is occasionally substituted for the shell. 
Burnt bone has been used to adulterate prepared shell; in this 
case, the solution in hydrochloric acid sometimes leaves a dark 
(nearly black) residue when filtered, and the filtered solution pre- 
cipitates copiously with caustic alkalies. 
Precipitated Carbonate of Lime. — This should be a pure white 
impalpable powder, tasteless, inodorous and insoluble in water; 
soluble in hydrochloric acid with effervescence, the bubbles break- 
