POT

POT

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It becomes gradually of a very dark brown purpofes, containing no eartb nor other foreign

colour, and is then effervefcent, though itfhewed no traces of carbonic acid before theboiling. The pure cryftals of the alkali whenfirnply heated, lofe their water of cryftalli-zation but do not acquire the property ofeffervefcing. But if thefe fame cryftals arediflblved in frelh alcohol, the folution whichwhen cold is clear and colourlefs, turns yellowas foon as boiling, by evaporation the colourdeepens, and even when highly concentratedit now refufes to cryftallize, and at laft whendeficcated, if water is added, though the alkaliis rediffolved, there remains a browniffi refin-ous infoluble matter.

The a£fcion of alcohol on potafh feems there-fore to be the following: it firlt difi'olves thepotafh, and feparates from it the greater partof the carbonated alkali which common potafhcontains, but a fmall portion of it is retainedtill the folution is concentrated, when the-more watery part of the alcohol feparates fromthe purer fpirit, and the carbonat of potafhfollows the former.

But befides this, a feparate a&ion is exertedby the potafh on the alcohol, whereby the fpiritis in part decompofed, and. furnifhes bothcarbonic acid and a blackifh refinous matter,and thus the alkali is again rendered flightlyeffervefcent, and the folution by diflblving thisrefinous matter becomes high coloured.

This gradual formation of carbonic acid bythe aftion of potafh on alcohol, which is notadverted to in Berthollet ’s original memoir, isfully proved by Pelletier, who fhews -that thefame change will take place even without heat,when the folution is kept for a confiderabletime, and the whole alkali is gradually changedto a carbonat.

From what has'been obferved, it will followthat the alcohol fhouid be allowed to remainonly a fhort time on the potafh., and the pureftpoflible alkali may be prepared fimply in thefollowing way: feledf a very pure carbonat ofpotafh, render at cauftic by lime, (leaving rathera little carbonic acid than an excefs of lime,)boil down the ley to drynefs, then add highlyre&ified alcohol, and boil the mixture for aminute or two, and fet it by in a well clofedveffel. Then pour off the alcoholic folution,evaporate it rapidly, and immediately bring itto perfect drynefs, which will leave a whitefaline rtiafs, fometimes covered with a blackcruft which may be removed, and the reft is aspure potafh as can be required for chemical

ingredient than a little of the refinous matterfrom the alcohol, which is of no importance inanalyfis. Or if abfolute purity is required, thecryftals of potafh feparated from the alcoholicfolution may be employed.

The method of allaying alkali for earthand carbonic acid has been mentioned, but itmay be right to notice a caufe of miftake whichmay occur. If to a very concentrated folutionof perfectly pure potafh in alcohol be addedlime water, or muriated barytes, or baryticwater, a precipitate will occur which mightlead to fuppofe that carbonic acid was prefent,but in this cafe the precipitate is pure lime orbarytes, and not a carbonated earth, which isfeparated from its folvent by the alcohol of thefolution, and is rediffolved on adding diftilledwater, which the carbonated earth is not.

In preparing cauftic potafh it fhouid beremembered that the folution of potafh at aboiling heat when highly concentrated, readilyacts upon glafs, and foon difi'olves a fufficientquantity materially to alter and impair its purity.In this cafe it gives a gelatinous precipitate withacids, which is the filex of the glafs. There-fore in preparing the alkali, all boiling andevaporating when the liquor is concentratedfhouid be performed in bright iron veffels, or(for perfect purity) in thofe of filver or platedcopper.

The alcohol diftilled off potafh carries upwith it fufficient alkali to turn fyrup of violetsgreen. d

Potafh is a fimple fubftance as far as hasbeen hitherto examined, though there is fomereafon for fulpecting it to be a compound.When folid and dry it is a white fait intenfelyacrid to the tafte, and corroding the fkin ofthe tongue ahnoft inftantly.

It exerts the fame action on every other partof the body, fo that when touched with themoiftened hand it has a greafy feel, owing tothe inftant aftion -which it begins upon thecuticle of the fingers. It is extremely foluble inwater, and is one of the moil deliquefcent iub-ftances known and hence it is ufefully employedin experiments to abforb moifture from any gasin which it is confined. Along with moiftureit abfo’-bs carbonic acid with great rapidity.

Potafh melts at a moderately red heat and thenruns into a clear liquid as thin as water, whichbecomes opake on cooling, and generally affumesa greenifh 'tinge, the caufe of which is not wellknown. At a full red heat (fuch as that of