TAR

TAR

( 407 )

them at once from the correfponding falts offoda and ammonia, which are not decompofedin the fame way.

By faturating the excefs of acid in cream oftartar with the feveral bafes (potafh excepted )various compound or triple fairs are produced,which will be mentioned prefently.

To obviate the inconvenience fometimesproduced by the very fparing fallibility ofcream of tartar, Lemery has recommended theaddition of borax. But as this latter fait con-tains an excefs of alkali, it is obvious that theacid of the tartar would be neutralized, and avery different fait would be produced, namely,the tartrite of potafli and 1'oda, or Rochellefait, which will be prefently defcribed, andwhich of itfelf is highly foluble without theaffiftance of borax. It appears however thatfimple boracic acids has the power of render-ing four times its weight of cream of tartarfoluble in as little as five or fix parts of hotwater, and probably without effecting any de-compofition of the tartar, fince the boracicacid is known to be remarkably weak in itsaffinity for the feveral bafes.

If a folution of cream of tartar in water isexpofed to the air for a length of time, it gra-dually becomes turbid, a number of mucousflocculi are depofited, and in the courfe offome months it ceafes to be acidulous, afterwhich it becomes fenfibly alkaline to the tafteand to chemical tefts, and it is finally con-verted into a weak folution of carbonat of pot-afh, the tartareous acid totally difappearing,and carbonic acid taking its place. Fire ope-rates a more rapid deftruftion of the tartareousacid, for if cream of tartar is calcined in anopen fire with a red heat, it firft foftens, black-ens, becomes of a pafty confiftence, the acidburns off with flame and fmoke, arid finally awhite carbonat of potafli is left. The alkaliprocured in this way is very pure, and is oftenobtained for the laboratory by moifteningcrude tartar or cream of tartar to the confift-ence of fliff pafte, wrapping up fmall parcelsof it in brown paper, and arranging them in agrate or furnace of any kind with charcoal, andkindling it. After'the charcoal has burnt out,the tartar is converted into lumps of carbonatof potafh, which ftill cohere, and may be rea-dily picked out of the afhes of the charcoal.A very pure carbonat of potafh may alfo bemade by deflagrating in a red-hot crucibleequal parts of nitre and cream of tartar.

This fait is compofed, according to The-

« taffone. i An. C

nard," of 57 per cent, of tartareous acid,

33 of potafli, the remaining 10 parts beingchiefly water of cryftallization. Of thefe 57parts of acid, 20 are in excefs, fo that thecompofition of the fait may be ftated, in a dif-ferent manner, to be 70 per cent, of tartriteof potafli, and 20 of tartareous acid. Themode of analyfiswill be mentioned in the nextfeftion.

Cream of tartar is decompofed by lime andbarytes, and probably by ftrontian, and caufticpotafli is left in the folution.

Tartrite of Potash, or Soluble Tartar.This fait, which is compofed of tartareous acidand potafh in mutual faturation, is preparedthe molt conveniently by adding cream of tar-tar to a hot folution of carbonat of potafh. Abrifk effervefcence takes place, and the additionof the cream of tartar fhould be continued tillthis ceafes, after which the folution fhould beboiled down till a pellicle appears on the fur-face, and then left to cryftallize by cooling.The tartrite of potafh then feparates generallyin the form of parallelipepids, with dihedralfummits. When this fait is prepared in the largeway for medicinal purpofes the evaporation iscontinued nearly to drynefs, with frequent ftir-ring, by which the fait is obtained in a fhapelefsgranular mafs.

Tartrite of potafh has a bitterifh faline tafte,is fomewhat deliquefeent, and is foluble inabout four parts of cold water, fo that in folu-bility it remarkably exceeds cteam of tartar.

It is partially decompofed by the ftrongeracids, which abftradf a portion of its alkali,whiift the whole of the tartareous acid remainsin union with the remaining alkali in the formof cream of tartar, as already mentioned in thepreceding feftion. Tartareous acid droppedinto a moderately ftrong folution of tartrite ofpotafh alfo caufes an immediate depofit ofcream of tartar.

Tartrite of potafh contains, according toThenard, 1 48 per cent, of tartareous acid, and43 of potafh, the reft, being chiefly water ofcryftallization. This he ellimates, iff, by heat-ing the fait to perfefit drynefs, avoiding its de-compofition •, 2d, by dilfolving the remainderin water, and decompofing it by acetited lead,and collecting the tartrite of lead thus pro-duced, (which laft by previous experiments, hefinds to con lift of 34 per cent, of tartareousacid, and 66 of oxyd of lead') •, and, 3d, byevaporating the remaining folution of acetite ofpotafh to drynefs, calcining it, dilfolving the

im. tom 41. 1 Ibid.