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ELEMENTS OF MATERIA MEDICA.

claim to the denomination of soda water (vide p. 191). Some fewmakers, however, prepare it by condensing carbonic acid in a solution ofcarbonate or bicarbonate of soda : this is soda water properly so called,formula; for which are given in all the British Pharmacopoeias , as well asin the French Codex ; but which I conceive are quite unnecessary, asthis liquid can be prepared extemporaneously by adding bicarbonate ofsoda to the ordinary bottle-soda water. Thus, half a drachm of bicar-bonate of soda and half a pint of bottle-soda water, will form a solutionof the same strength as the liquor soda effervescens of the London Phar-macopoeia, which is directed to be prepared by dissolving a drachm ofsesquicarbonate of soda in a pint of distilled water, and forcing into itan excess of compressed carbonic acid gas. The aqua carbonatis so dmacidula (Ph. Dublin) is of the same strength: the aqua super-carbonatissodce (Ph. Edinb.) contains 96 grains of carbonate of soda to a pound ofwater.

A fraudulent imitation of soda water is said to have been practised byadding a few drops of sulphuric acid to a solution of carbonate of sodain water, and instantly corking the bottle. The fraud may be detectedby chloride of barium, which throws down a white precipitate insolublein nitric acid.

Soda water properly so called ( liquor soda effervescens, Ph. L.) isemployed in the same cases as bicarbonate of soda. The additionalquantity of carbonic acid contained in it, renders it more agreeable andnot less effectual, as an alkaline agent, in its operation on the systemgenerally. It is employed to counteract or prevent the inordinate secre-tion of uric acid and the urates; but both this and soda water powdersare highly injurious in phosphatic deposits,—in the latter case, however,carbonic acid water (ordinary bottle-soda water) may be used (videp. 198).

Potas'scB So'dio-Tar'tras. — So'dio-Tar'irate off Pot!ash.

History. —This salt was discovered by Seignette, an apothecary atRochelle, in 1672, and hence it is frequently termed Seignette's salt orSel de Seignette (Beckmann’s Hist, of Invent, vol. iv. p. 616). He calledit alkaline salt, sal polychrest, and Rochelle salt (sal rupellensis). Todistinguish it from the sal polychrest (sulphate of potash) of other writers,it is sometimes denominated sal poly chrestum Seignetti. Its other namesare, tartarized soda (soda tartarizata seu natron tartarizatum ), or thetartrate of potash and soda. In the Pharmacopoeia it is termed sodcepotassio-tartras.

Preparation. —The following are the directions for preparing thissalt in the London Pharmacopoeia:—Dissolve 12 ounces of carbonate ofsoda in four pints of boiling water, and add, gradually, 16 ounces ofpowdered bitartrate of potash. Strain the liquor; then apply a gentleheat until a pellicle floats, and set aside, that crystals may be formed.The liquor being poured off, dry these. Again evaporate the liquor, thatit may yield crystals.

In this process the excess of acid in the bitaxtrate of potash is satu-rated by the soda of the carbonate, while the carbonic acid of the latteris disengaged.

Properties.— This salt is met with in large, transparent, and regu-