Gfiici-al Refults oficei-ning Alum. 32 j 



pedes the cryftallizatlon of a!um, but it is the ?.-ant of pot-aP.i or ammoniac. For it 13 

 difficult indeed to imagine that the fulphuric acid could remain difengaged after fo long re- 

 maining upon alumine in a (late of extreme divifion, and always fuperabundant. (t is true 

 that the aluminous waters redden the vegetable tinftures, but this property is not owing to 

 a difengaged acid. This portion of acid is a conftituent part of thefe waters, and it appears 

 to have more affinity with the neutral fulphate of alumine than with a new quantity of 

 this earth at the temperature of the atmofphere. 



2. The fulpliate of pot-afli may be ufed, as well as pure pot-afli, to caufc the crydalllza- 

 tion of alum. It even has the advantage over the latter fait, becaufe, if the alumi- 

 nous waters do not really contain a difengaged acid, the pot-afli, in its coinbiration, will 

 precipitate a portion of alumine, and diminifli the produifl: of the boiling ; whereas the ful- 

 phate of pot-afli does not produce the fame efFecfl : but if the llxiviums contain difengaged 

 acid, which mull very feklom be the cafe^ it is not converted Into alum by the fulphate of 

 pot-afli, and is loft with regard to the produ£l. I think, therefore, that with regard tofuch 

 waters as really contain an exccfs of acid, or a very oxidcd fulphate of iron, the ufe of pot- 

 afh may be preferable to that of the fulphate of pot-afli. But with regard to the price of 

 thefe fubflances, I thin4i that in many places it would be profitable to ufe the fulphate of 

 pot-afh, becaufe it is a fait indiredly produced in a great number of manufacfories, where 

 it may of courfe be obtained at a very modernte price. The refidues of the diflillation of 

 aqua fortis by the fulphuric acid would be excellent for this operation. I mean that the? 

 would be preferable to the neutral fulphate of pot-afh. For I have remarked that this lalt 

 precipitates a portion of aluminous earth, which the other does not. This fall would more 

 cfpecially polTcfs a decided preference before pot-afli, in thofe cafes where the aluminous 

 waters contain at the fame time a great quantity of fulphate of iron intended to be ufefully 

 employed, becaufe it would a£l immediately on the fulphate of alumine, without touching 

 that of the iron ; whereas the pot-aih does not begin to form alum until the whole of the 

 ferruginous f.ilt is deconipofed. It would be in particular of much greater advantage tha:i 

 putrid urine, becaufe this fluid always contains phofphoric falts, which decompofe a portion 

 of the fulphate of alumine, and confiderably diminifh the produft. 



This inconvenience might, hov, ever, be avoided by adding a certain quantity of lime to 

 the urine, to precipitate the phofphoric acid. 



3. Alumine cannot be ufed in the treatment of mother waters, as Bergman propofcs. 

 This earth is incapable of favouring the cryfliallizatlon of alum, befides which, it decom- 

 pofcs a portion of alum by the alhftance of ebullition ; In which circumftance it feizes the 

 acid neccflary to its folution, and precipitates it in the form of that powder which is called 

 alum faturited with its earth. 



4. Many alum ores mull naturally contain pot-afh, becaufe perfed alum is often obtained 

 from the firll cryftallizatlon of new alum waters without the addition of this alkali. It 

 is true that anobjciSlion may be made with regard to the wood ufed in calcining thefe 

 ores, which may be fuppofcd to have fiirnillied the alkali ; but it is not probable that the 

 fmall quantity of wood employed, in comparifon to the quantity of ore and the alum it af- 

 fords, could fupply enough of jtot-afh for the cryftallizatlon. 



5. All the earths and ftonen \Vhich have given, or fliall hereafter afford, by analyfis with 

 the fulfUuric add, pcrfeiSl alum without addition of pot-afh, muft contain this alkali na- 



T t 2 turallv. 



