ANALYSIS OF LITIIOMARGE. 169 



jn hour, and the fediment collected proved to 

 be the argillaceous earth, exactly faturatcd with 

 vitriolic acid. Alum, it is. well known, requires 

 an cxccfs of acid ; without which it lofes its fo- 

 lubility with its other properties. Of this excefs 

 it may be deprived in various ways, and the ar- 

 gillaceous earth precipitated, cxaclly faturated, 

 but iafol.ublc *. If tlic fedimcnt is fufliciently 



digcRcd 



* Tlic cxccfi of aud in alum may he dellroycd by alkali, 

 iinu 1 , magncfia, pure argillaceous earth, iron, or /inc. la 

 the lii'll volume of thcfc lilf.iys, pn;^e 334, aerated chalk j* 

 employed to fcparatc alum from vitriolattd ma^ccfia ; and this 

 medium never fails unlcfs its fucccfs is purpofely obfiatcd. 

 If any one, with an intention to miflead, fhould dilute llic 

 folution too imch, and ufe largo pieces of the chalk inllead of 

 its powder, lie will certainly be difappointed in his ddign, for 

 fcurce any fenfiblc eflecl will be produced. Too gnat a quan- 

 lity of w.itcr removes the particle? of alum fo far from any 

 contjifi with the chalk, that no dccompofition can lake place. 

 I>etiiicM the pieces employed pufcnl a much fmullcr furfacc 

 than when they are reduced to powder. Whoever h really 

 bent on [jiviii;^ a fallacious appearance to this experiment, 

 will not confider the remarks made here as worthy his atten- 

 tion. But fhouM any one be defirous of certain conviction, 

 let him mix a known weight of r.lum and vitriolatcd ma^ncfia, 

 ;*nd diifviiVC them in as much tepid water as is neeeiTary, and 

 add aftcnvards gradually fmall portions of powdered chalk un- 

 til fueli time as the folution ccafcs to redden the tindure of 

 turnfolc. If the cfied ia required very foon, cxpofe the fo- 

 lulion to a iUong digcftin^j heat, othcrwife a lef^ decree of 

 temperature will be fuificient. The acid bcin<; thus dell roved, 

 pafa tiie lijtjor through a filtre, and wafli the inaf* with pmc 

 water ; evaporate the whole of the liquor that paflcd through 

 tin- filt re to i!r-ncf, aud the rcfult will be the true weight of 



vitfiolatcd 



