Lond. Supertartras Potassae, Edin. Potassae Bitartras seu Tartari Crystalli, Dub. Supertartrate or Bitartrate of Potassa, Crystals of' Tartar.

Syn. Tartrate acidule de Potasse (F), Weinsteinrahm ( G.), Wynsteenroora (Dutch), Cremore di tartaro; ossitartrato ossidulo di Potassa (I.).

This is the saline crust deposited on the sides of casks of wine, purified. It is first reduced to powder, then dissolved in boiling water in tubs, and the clear fluid poured off from the sediment. The clear solution is then allowed to remain at rest, when it deposits brown crystals of tartrate of potassa, which are boiled in copper vessels with the mother liquor; and clarified by throwing in whites of eggs, and some finely sifted wood-ashes. An effervescence immediately takes place, and a red scum is thrown up, which is carefully skimmed off with a perforated skimmer; and the throwing in of the wood-ashes, with the subsequent skimming, are repeated for fourteen or fifteen times; after which the liquor is taken from the fire, and allowed to remain at rest for three days. On the fourth day, a dirty white saline crust is removed from the surface, and two thirds of the liquor ladled out. The crystal's which now form are white and clean, and require no farther preparation than drying on a wicker frame. In some places, instead of wood-ashes, a portion of pure clay is diffused through the boiling solution.

The exposure of the crystals on cloths to the air and light whitens them very considerably.2

Qualities.-Bitartrate of potassa is inodorous; and when allowed to dissolve in the mouth, which it does very slowly, and feeling gritty under the teeth, has a harsh acid taste. Its crystals are small and irregular six-sided prisms, generally run together into little masses, which are of a white colour, semi-transparent, brittle, and easily reduced to powder. Its specific gravity is 1.953. It requires for its solution 30 parts of boiling water, and 120 of cold water. The solution decomposes spontaneously by keeping : a mucous matter is deposited, and there remains a solution of carbonate of potassa, coloured with a little oil.1 According to the analysis of Thenard, 100 parts of pure bitartrate of potassa contain 57 of tartaric acid, 33 potassa, and 7 of water2: according to Berzelius, the proportions are, acid 70.45, potassa 24.80, and water 4.75; while Mr. Brande states them to be, acid 73.64, potassa 26.4. In equivalents its constituents are 1 eq. of potassa = 47.15 + 2 tartaric acid = 132.96 + 1 water = 9, equivalent 188.96. Mr. Brande remarks that it is doubtful whether this salt contains any definite water of crystallization.3

1 In Iceland the recent root of bistort is eaten raw, or converted into bread. It may, therefore, be reasonably enquired, What effect can it have as a medicine, when prescribed in the small doses visually ordered?

2 Schauh says, it may be purified by simply boiling it with powdered recent charcoal, and very white crystals obtained. Annates de Chimie, xlix. 64.

Medical properties and uses.-This salt is purgative and refrigerant. As a purgative it is frequently employed, on account of its taste being less unpleasant than the generality of saline cathartics : but it is apt, when long used, to produce emaciation, chiefly owing to its powerful action on the intestinal exhalants. This property is taken advantage of with great effect in the treatment of dropsy, particularly ascites; in which the bitartrate of potassa has been found extremely efficacious. It occasions a considerable discharge of serous fluid into the bowels, which is thrown off in the form of Watery stools, in which case the discharge by urine is not augmented. The water in the cavity of the abdomen is, however, thus rapidly carried off; and the chances of a return of the disease are supposed to be fewer than when diuretics are employed. We are of opinion, however, that in cases complicated with hepatic obstructions the effects of this remedy are very uncertain. It may be advantageously united with squill; and, owing to the exhaustion it occasions, its use should be followed by preparations of iron, and other tonics.

As a refrigerant, bitartrate of potassa dissolved in water, and the solution sweetened with sugar, is a pleasant beverage in febrile diseases, when its purgative quality is not likely to prove injurious. As a purgative and hydragogue the dose is from 3 iv. to 3 vj., in the form of electuary; but this dose is too large in ascites : not more than Э j. is necessary.

Officinal preparations.-Ferrum Tartarizatum, L. D. Potassae

Tartras, L. E. D. Antimonium Tartarizatum, L. E.D. Soda

Tartarizata, L. E. D.

1 This decomposition was first described by Berthollet, in 1782. Mem. Far. Thomson's Chymistry, 4th edit. vol. iii. p. 93.

2 Annales de Chimie, vol. xxxviii. p. 39.

3 Manual, p. 142.