in the Produ&ion of Alum, ^j 



precipitated argillaceous earth, from which it may be feparated by the known methods, if 

 it fliould be required to afcertain the quantity of the argil. A certain portion of the ful- 

 phate of lime, thus produced, is likewife precipitated at the fame time, and in greater 

 quantity, the lefs water there is in the folution. The remainder of the gypfum continues 

 diffolved in that fluid. Let the liquor be carefully poured off from the precipitate, and 

 let this laft be edulcorated with a fuflicient quantity of boiling water ; let the walhings be 

 added to the fluid at firft decanted ; and the whole of the fluid be gradually evaporated, 

 and left to cool, by alternations. By this management the gypfum will by degrees be fepa- 

 rated, and afterwards the fulphate of alkali, which was an ingredient into the alum, 

 whether it has been pot- alh, or ammoniac. The fulphate of lime will fall firft, becaufe 

 moft difficultly foluble ; the fulphate of pot-afh will be the next in order; and, laftly, the 

 ammoniac, being the moft foluble of all. If at the fame time the alum under examination 

 has contained a portion of fulphate of foda*, it will be more difficult to feparate this from 

 the fulphate of ammoniac, becaufe it is alfo very foluble. But it will prefently be fliewn, 

 that no foda is contained in alum. 



3. Pour into a faturated folution of alum, in fix times its quantity of water (heated to 

 about 120° Farenh.) a faturated folution of acetate of lead. Sulphate of lead will be pro- 

 duced, which falls down ; and alfo, the acetate of alumine as well as the acetate of pot-alh 

 (or of ammoniac), which, being readily foluble, remain diffolved in the water. The 

 fupernatant fluid muft then be decanted from the fulphate of lead, and fubjefted to eva- 

 poration, until it beomcs turbid ; for by this means a little of the metallic fulphate, that was 

 ftill held in folution by the water, is thrown down. When the fluid has been a fecond 

 time filtered, it muft be evaporated to drynefs. The dry refiduum confifts of acetated argil 

 and acetated pot-afli. And if it be then ignited,' the acetic acid, together with the am- 

 moniac, if prefent, will be volatilized. Water muft now be poured upon the refidue; 

 which, if only ammoniac has entered into that compound, extradls nothing from it, by 

 xeafon of the volatilization of this alkali, and of the infolubility of aluminous earth in 

 water. But if a portion of vegetable alkali was contained in it, it is on account of its 

 fixity, left behind in the argil, and will be diffolved in the water. The alkali receives 

 fome carbonic acid from the deftruftion of the acetic, but is ftill far from being faturated 

 with it ; and hence a part of the argil will be diffolved along with it by the water. The 

 earth muft, in that cafe, be precipitated, by faturating the folution with carbonic acid, or 

 letting it ftand for a long time in open veffels, flightly covered. 



Analytical experiments alone are, neverthelefs, infufficient to decide with certainty, 

 whether all the three alkalis, or only one, or two, of them are by their acceffion capable of 

 producing alum ; and therefore our arguments cannot yet reft on a complete indudlion. 

 For, fuppofing I had decompofed artificial as well as native alum in a thoufand different 

 ways, and had found no foda in it, I cannot fairly conclude that the mineral alkali is 



* I need not here obferve, that falts of more difficult folubility are, for the moft part, (though not per- 

 feftly) feparated by the gradual and fucceffive diminution of the water, and cryftallization. 



unfi t 



