POT

POT

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melting copper) potafh is readily volatilized, andflies oft in a denfe vapour which is eafily perceivedwhen a crucible holding it thus heated is fudden-ly uncovered. This vapour is potafh unaltered.

Potafh diflblves all earths with extremefacility by fufion, and even in watery folutionwith the affiltance of heat. The mixture ofpotafh and earth is loluble in water when thealkali is in excefs, and the folution depofits theearth when faturated by an acid.

Potafh when folid and deprived of its waterof cryftallization produces a fenfible heat whenmixed with water; but the cryjlals of potafhform with fnow or powdered ice one of themoft powerful freezing mixtures that is known,and which is defcribed under the article Freezing.

Potafh unites with all acids, producing fails-that are defcribed under thefe articles reflec-tively or thofe immediately fucceeding. For thegeneral properties which it poffeffes fee Alkali :and for its aftion upon the earths in the moiftor dry way upon Oils, Sulphur , Phofphorus, feethefe articles and thofe of Soap and Glafs.

Potafh when highly concentrated is very aptto caufe glafs phials in which it is kept to crack.It alfo feems to contract a very clofe unionwith glafs in any form, fo that the drop or twowhich wets the fpace between the ftopper andthe neck of the phial; will after a while cementit together fo llrongly as to require much painsand perfeverance to feparate them unlefs pre-vioufty oiled.

An impure potafh melted and call: into'moulds, is ufed in furgery as a valuable andpowerful cauftic. It is mixed with foap, andapplied in the form of a foft pafte to the fkin,and in fome hours it deftroys the life of thepart to which it is applied, which portion aftera while comes out in the form of a deepftough.

This alkali refembles foda fo clofely in tafte,appearance, and general chemical properties,that they are not readily diftinguifhable in anyother way than by uniting them with thefeveral acids and examining the neutral faltsrefulting from this combination. The faltsthat are moft readily diftinguifhable are thefulphats, nitrats, and tartrites. Thus the ful-phat of potafh is a hard bitter fait, requiringmuch water for folution, and therefore a falineprecipitate of fulphat of potafh is formed whenthe acid and alkaline folution are fomewhatconcentrated when added: whereas the ful-phat of foda is very foluble, and differs effen-tially in its form of cryftallization from the•ther. The nitrat of potafh cryftallizes in long

fix-fided prifms, but the nitrat of foda in cubes.The acid of tartar alfo is a very ufeful teft. Ifthe alkali to be examined is firft diffolved inmuriatic or any other acid, and a folution oftartareous acid added, if the fait has potafh forits bafis, there will be a precipitate of tartar ina few feconds, but the mixture will remainclear if the bafis of the fait is foda. Thenitro-muriat of platina alfo decompofes all faltswith potafh, but not with foda.

The affinities of potafh are in the followingorder: the fulphuric acid, nitric, muriatic,phofphoric, fluoric, oxalic, tartaric, arfenic,fuccinic, citric, fulphureous, acetic,, carbonicand pruflic.

This order however, as in other cafes ofaffinity, is to be taken with very confiderablelimitations, and in particular does not applywhere the fame acid forms both an acidulousand a neutral fait with potafh, as in the cafe ofthe tartareous juft mentioned, which will par-tially decompose even fulphat of potafh, that is,to the point at which the acidulous tartrite isformed, but no further.-

POTTER’S LEAD ORE is the term ufuallygiven to the common galena. See Lead.POTTERY and PORCELAIN.

Our limits would not allow us (even if wehad the means 1 to enter into any detailed de-feription of the antient and beautiful and moftufeful art of pottery, efpecially as the mofteffential part of it can hardly be called a chem-ical operation, though, coniiderably connectedwith chemiftry in many of the fubordinate parts.We fhall therefore give but a fingle example ofthe procefs by which a lump of plaftic earth isconverted into a hard brittle vefl’el, and add fomeobfervations on that part of the manufacture,which is peculiarly deftined for chemical ufes.

The effential material of pottery is clay,which alone poffeffes the two requifite qualitiesfirft, of being in its natural ftate fo plaftic thatwith water it becomes a foft uniformly extenfiblemafs, capable of taking and retaining any form,and fecondly, that when it is thoroughly driedand has undergone a red heat for a time, itlofes this plafticity, is no longer retentive ofwater to any confiderable degree, becomes hard,clofe in texture, and able more or lei's perfectlyto retain all liquids contained within its hollow.

The various compofition of Clay has beendefcribed under that article and Alumine , and ithas been there fliewn, that though clay owesits plafticity to alumine, it feldom contains fomuch of this as of other earths, and is in fact ,a very compound fubftance.

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