Rochelle Salt

Rochelle Salt is the popular name of the tartrate of soda and potash ($\backslash text\{KNaC\}\_4\backslash text\{H\}\_4\backslash text\{O\}\_6\; +\; 4\backslash text\{H\}\_2\backslash text\{O\}$), this salt having been discovered in 1672 by a Rochelle apothecary named Seignette. It occurs, when pure, in colourless transparent prisms, generally eight-sided; and in taste it resembles common salt. It is prepared by neutralising cream of tartar (bitartrate of potash) with carbonate of soda. After a neutral solution has been obtained, it is boiled and filtered, and the resulting fluid is concentrated till a pellicle forms on the surface, when it is set aside to crystallise. This salt is a mild and efficient laxative, and is less disagreeable to the taste than most of the saline purgatives. From half an ounce to an ounce, dissolved in eight or ten parts of water, forms an average dose. A drachm of Rochelle Salt added to one of the ingredients of an effervescing draught (bicarbonate of soda or tartaric acid, for example) forms one of the varieties of what are called Seidlitz powders.