Purging Salt: a salt of a bitter taste; soluble in twice its weight or less of water, and shooting into long prismatic crystals like those of nitre; liquefying and bubbling up in a moderate heat, emitting a large quantity of aqueous vapours, and changing to a white spongy mass, bitterer than the salt at first. It is of two kinds; one a combination of the vitriolic acid with the earth called magnesia; the other, a combination of the same acid with the fixt alkali called natron. The most obvious criterion of their acid being the vitriolic is, their precipitating chalk dissoblved in aquafortis or in other acids.

1. Sal amarus Pharm. Lond. Sal cathar-ticus amarus Pharm. Edinb, Nitrum calcareum Listero & hydrologis quibusdam. Purging bitter salt: composed of the vitriolic acid and magnesia; distinguishable from that whose basis is an alkali, by solutions of it being turned milky, and depositing their earth, on the addition of any alkaline salt.

This is the salt with which the purging mineral waters are principally impregnated, and on which their purgative quality depends. It was first extracted from the Epsom waters, and has been commonly distinguished, both in this and other countries, by the name of Epsom salt: but those waters yielding the salt very sparingly, and their quantity being insufficient for its great demand, it was fought for else-where, and found, in plenty, in the bitter liquor remaining after the crystallization of common salt from sea water; from which it is now generally prepared.

This salt is a gentle purgative, operating in general with ease and safety, yet with sufficient efficacy, and quickly finishing its operation: its palling off hastily, and not extending its action so far as most other purgatives, seems to be its principal imperfection. For a full dose, eight or ten drams may be dissolved in a proper quantity of common water, or four or five drams in a pint or quart of the purging waters; to which may be added a little tincture of cardamom seeds, or some other grateful aromatic, to render the liquor more acceptable to the stomach. These liquors, in smaller doses, pass further into the habit, promote the secretions in general, and prove excellent aperients in fundry chronical disorders.

2. Sal catharticus glauberi, vulgo sal mirabile. Glauber's cathartic salt: composed of the vitriolic acid and the mineral alkali natron, and hence suffering no change from an admixture of fresh alkali.

This salt was discovered, by the chemist whose name it bears, in extracting the acid spirit of sea salt by means of the vitriolic acid. When oil of vitriol is poured on sea salt, the marine acid, thereby disengaged from its own alkaline basis, begins immediately to exhale, and by applying heat may be totally expelled; the vitriolic acid remaining combined with the natron or marine alkali. This combination is still procured chiefly in the same manner: to the sea salt is added six tenths † or half ‡ its weight of oil of vitriol diluted with water, and the marine acid being distilled off, the residuum is dissolved and crystallized. The smallest of these proportions of oil of vitriol appears to be sufficient for expelling the acid and saturating the alkali of the sea salt; but the larger is more eligible, as the Glauber's salt does not well crystallize unless the acid prevails in the solution.

This salt is nearly of the same medicinal qualities with the foregoing, which frequently supplies its place in the shops. The Glauber's salt, somewhat the least unpleasant to the taste, is supposed to be the mildest of the two, and to operate the most kindly.

Natron vitri-olat. f Ph. Lond.

Soda vitrio-lata vulgo fal cath. glauberi I Ph. Ed.