TAR

TAR

( 404 )

may be fomewbat doubted, for tbe fuper-tartriteof potafh appears in alt experiments to be apretty uniform fait; and its little folubility, andthe readinefs with which it feparates from itshot faturated folution, contribute to preferve itscompofition unvaried in any mixture whereit is produced at all.

In the Montpellier mode of purification theearth ufed appears to be a tolerably pure clay,which readily diffufes itfelf in water, and pro-bably any other white clay would anfwer aswell. The effedt of the clay is to abforb to it-felf the mucilaginous and coloured extract, andto carry it down in the form of an infolublefediment; and when this operation is properlyperformed it appears on the whole to be fu-perior to the Venetian mode. The clay how-ever fhould be free from any admixture ofchalk, and Ihould not effervefee with acids', forthe chalk would readily neutralize that excefsof acid which is eflential to the compofition ofcream of tartar, as is feen in the procefs whereit is intentionally added to procure the tartareousacid, and will be deferibed under that article.

Cream of tartar is a very valuable fait. It ischiefly ufed in dyeing, efpecially in giving thefcarlet and other modifications of the cochinealcolours as deferibed under the article Dyeing. Itis alfo frequently combined with alum, as amordant in fixing colours. An agreeable andvery cheap acidulous drink is made by difi'olvingthis fait in water. The other chemical pro-perties of this fait will be mentioned under thearticles Tartrites andTARTAREOus Acid.

TARTAR EMETIC. See Antimony.

TARTAR Regenerated. . See Acetite ofPotafi.

TARTAR. Salt of. A very pure Carbonat ofPotafi is made by calcining cream of tartareither perfe or with nitre, whence the term Saltof tartar has long been, and ftill is, very com-monly applied to the pureft fub-carbonat ofpotalh in whatever way obtained, and fome ofthe falts with this alkali have had the name oftartar, as the following:

TARTAR Vitriolated. See Sulpiiat of Pot-afi.

TARTAR Soluble. See Tartrite of Potafi.TARTAREOUS ACID.

It was Scheele , whofe admirable ingenuityfirft devifed the method of feparating fromcream of tartar the pure- tartareous acid, andobtaining it in a folid cryilallized form.

His procefs, with flight variation, is the fol-

lowing, which very elefely refembles the me-thod ufed by this chemift to obtain the Citricacid: diflolve any given quantity of cream oftartar in boiling water, and whilft boiling addgradually lbme clean powdered chalk. A copi-ous efFervefcence will arife, and the addition ofchalk mult be continued till this ceafes, whenthe mixture may be fet by to cool. It thencontains a white denfe fediment, which confiftsof the lime of the chalk, united with the excefsonly of the acid of the cream of tartar; andthe fupernatant liquor is therefore a folution ofthe cream of tartar deprived of its excefs of acid,or neutral tartrite of potafi , or foluble tartar as itis alfo called, and which may be obtained cryf-tallized by fubfequent evaporation. Wafh theprecipitated tartrite of lime repeatedly with coldwater, then put it into a glafs veflel, and addto it a diluted fulphuric acid, compofed of asmuch concentrated acid as is equal to theweight of chalk employed in faturating thecream of tartar, mixed with four or five timesits weight of water.

The fulphuric acid having a ftronger affinityfor the lime than the tartareous acid has, totallydecompofes the tartrite of lime during a di-geftion of two or three days (or in a {hottertime if affifted by a gentle heat) and the whitefediment, though it does not alter its appear-ance, is changed to fulphat of lime, whillt thefupernatant liquor contains naked acid of tar-tar. Then pour off the clear liquor, wafh thefulphat of lime to extract all the adhering acid,and add the waffiings to the former liquor, andevaporate the whole (at firft with a boilingheat, and as it concentrates, with a muchgentler warmth) till it is of a thick fyrupy con-fidence, and then fet it by for fome hours thatall the felenite which it may hold in folutionmay be depofited. Then again dilute the mix-ture with cold water fufficient to rediflolve everything but the felenite, and llowly evaporate thefolution to a fyrupy confiftence, and after fomehours it will depofit the pure tartareous acid incryftals, which are generally pretty large irre-gular hexahedrons. Cream of tartar decom-pofed in this way by chalk (and therefore onlypartially) will yield about a third of its wfightof the cryftallized acid. This quantity how-ever mu ft not be taken as the proportion of theacid in cream of tartar, for much of the weightof the cryftallized acid is water of cryftalliza-tion, whereas the cream of tartar contains verylittle water.

Some nicety of management is required toobtain the utmoft quantity of the acid and with