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admixture of carbonic acid gas with azot whenthe latter is paffed through red-hot porcelaintubes, which does not take place in glafs tubes.The carbonic acid therefore arifes from theburning fuel, and pafles through the pores ofthe tube when red hot.

POTSTONE. Topfjiein, Germ. Pierreellaire, Fr.

The colour of this mineral is greenifh grey.It occurs in mafs. Its internal lultre is gliften-ing and pearly. Its fra£ture is curved, foliated,and imperfectly flaty. Its fragments are tabu-lar. It is often tranflucent on the edges. It isfoft, is greafy to the touch; tough and diffi-cultly frangible. It is found in beds with Ter-pentine , at Como in the Grifon country, andin Saxony . It may be turned in a lathe, and ismade into a variety of veilels. It is alfo ufedas a lining for furnaces, being remarkablyrefractory.

POWDER OF CASSIUS. See Gold.PRASE. See Quartz.

PRECIPITATE. Niederfchlag, Germ.

A chemical precipitate properly means avifible fubftdence of any fubftance from its clearfolution, produced by any method whatever,and it is generally applied when the reparationtakes place in a flocculent or pulverulent form,in oppofition to crystallization, which implies afimilar feparation in an angular polyhedral form.Thus even a mafs of cryftals may be termed aprecipitate, when their fubfidence is fo fuddenthat their proper cryftalline fhape cannot bediftinguifhed by the naked eye, as whenGlauber’s fait is fuddenly feparated in veryminute cryftals from its faturated watery folutionby the alfulion of alcohol.

But precipitation may be taken in a moreextended fenfe to mean any vifible feparation ofany fubftance or compound, from its clearfolution, whether it finks or fwirns, and whetherit be cryftallized or pulverulent, and it thenforms one of the great operations in. chemiftry,and is directly oppofed to that of folution; forin fact by far the greatell part of the operationsof the chemift may be refolved into folutionand precipitation.

The mode in which precipitation affe£ts andmodifies chemical affinity, has been mentionedunder that article, and Berthollet has fliewn inhis valuable effays on chemical affinity and onchemical ftatics, how philofophers have been ledto deduce the fuppofed order of affinities fromthe circumftances of precipitation.

Tae following remarks on the precipitations

effected by acids or by alkalies and earths maybe added from the fame author . 1 The impor-tant lubject of metallic precipitates will benoticed feparately.

There are two principal kinds of precipitateswhich have been diftinguilhed by chemiftry,one where to a compound compofed of an acidand a bafe, each highly foluble in water,another acid is added, which has a greateraffinity with the bafe, and forms with it acompound lefs foluble than the firft. In thiscafe the precipitate (which is fometimes cryftal-lized, fometimes otherwife) is compofed of theacid laft added, in proportions that vary butlittle. As for example when fulphuric acid isadded to a faturated folution of nitrat of potaflia cryftallized precipitate of fulphat of potafli isadded, the proportions of which fcarcely vary inany circumftances, though the actual quantityof precipitate is determined by the quantity ofwater and of fulphuric acid prefent.

Another kind of precipitate is rhat which hasufually been fuppofed to be fimple, and takesplace when to a compound of an acid and abafe infoluble (or nearly fo) in water, analkali or foluble earth is added, which byelective affinity unites with the acid, anddifplaces the former bafe; and this latter nowfuppofed to be uncombined and infoluble in .thequantity of liquid prefent, falls to the bottom.This kind of precipitation is conftantly refortedto when we with to obtain the fimple earths, as inadding an alkali to a folution of alum in order toprecipitate the alumine, to Epfom fait to preci-tate magnefia, &c. But the precipitate in allfuch cafes is not (as has been ufually fuppofed)the fimple earth, but it always retains after thefirft operation a fmall and variable portion ofthe acid with which it was before combined.Thus if the alumine, however well edulcorated,be rediflolved in muriatic acid, and a fait ofbarytes added, there will be a fenfible precipi-tate of fulphat of barytes. This retent of acidis variable and may be reduced to an infenfiblequantity, if not abfolutely taken away, -byfubfequent digeftion in very concentratedalkali. Such precipitations therefore are noteffected merely by fimple affinity, but by adivifion of the acid into two portions, oneof which, and by much the largeft, goes to thealkali, and the other remains with the earth andaccompanies it in its precipitation. Even whenno precipitation occurs, the acid may be provedto diftribute itfclf in this manner, for asBergman has remarked, if a calcareous fait is

^Chemical ftat-ics. \ol. I.