314 Central Rfftillt concerning Alum. 



turally. For it is well proved, by the experiments related above, that alum cannot exiO 

 without pot-afli or ammoniac ; and as there is little probability that this laft fliould be found 

 combined in earths or Hones, unkfs perhaps in: very rare cafes, we may almoft conftantly 

 be allured, when alum is cbtaini.d from any of thefc fiibflanccs, that its formation was ef- 

 fcfted by pot-a(h. The quantity of alum will immediately fliew in what proportion this 

 alkali exifled in the fubllances analyfed. I have announced to the InRitute in the com- 

 munication of my experiments on the leucite, that I had begun a feries of experiments on 

 fcver.il earths and ftoncs which I prcfumcd to contain pot-ad). At prefent I can give the 

 rcfults of fome of my experiments. 



The crude alum ore of La Tolfa aflorded me 2,3 per cent. ; but as it Is difficult to ex- 

 tract more than two- thirds of this fubftance from the ftoncs M-hich contain it, we may, 

 v/ithout fear of mlftake, eftimate tlie quantity at 3,4 per quintal *. The zeolite of the Ille 

 of Ferro afforded 1,78, which makes according to our eRImatlon 2,37 per cent. The ar- 

 gillaceous earth of Forges in Normandy, of which the pots of the gl ifs works at Seves 

 arc made, likevvife afforded It, but in a very fmall quantity f . The adamantine fpar, on the 

 analyfis of which Citizen Guyton is at prefent employed, niuft contain a confiderable 

 quantity ; for it alTords much alum when treated immediately with the pure fulphuric 

 acid. In reading over the analyfes of (lones which have hitherto been made, an almoft 

 certain proof will be found, no doubt, either by the lofs in the fum of their produdls, or 

 bv the alum they afforded with the fulphuric acid, that they contained potalh, though no 

 mention either was made or could be made refpe£ling it. 



6. The alum of commerce ought not to be confidered as a fimple fait, but as a com- 

 fcinatlon in the ilate of a triple and fometimes quadruple fait of fulphate of aluminc, ful- 

 phate of pot-afti, or of ammoniac. Among thefe lad we may diftinguifli two fpecies ; the 

 one without excefs of acid, infoluble in water and infipid, being what is improperly called 

 alum faturated with its own earth ; and the other, which contains an cxccfs of acid folubic 

 in water, very fapid and aflringent, is the common alum. 



There is likewife a pure fulphate of alumine, very aftrlngent, very difficult of cryftalllza- 

 tion, in the form of brilliant pearl-coloured plates without confidence, and which cannot 

 be rendered infoluble by the addition of a new quantity of its bafe. This lad ii\t may with 

 the greated propriety be called the fulphate of alumine. 



7. It follows from the comparative analyCs, and the knoviledge acquired rcfpcfling the 



didcrenl 



