Anol^'s of the Alumtf Commerct, 321 



In fadt, if-thls chemlft had thought pot-afli eflential to the conftitutlon of alum, he certainly 

 would not have thought it advlfable to ufe a portion of the clay of Cologn to deftroy the 

 excefs of acid in aluminous waters. For this addition is extremely vicious, and I am con- 

 vinced that it was fuggefted by reafoning rather than by experiments *. 



Out of all the kinds of alum which 1 had fubmitted to analyfis, I did not find one which 

 did not afford fulphate of pot-afli or of ammoniac, and frequently both at once. I ufed the 

 following method of analyfis : In the firll place, to dire£l my operations, I take a fmall 

 quantity of the alum I wi(h to afTay, which I difTolve in the aqueous folution of pure 

 pot-a(h, and flightly heat the mixture. If it contain fulphate of ammoniac, a ftrong fmell 

 of volatile alkali is immediately perceived. I then put into a tubulated retort a given quan- 

 tity of this fulphate of alumine ; I adapt a receiver containing a fmall quantity of water, and 

 then pour on the alum a folution of pot-a(h in a proper quantity to decompofe the ful- 

 phate of ammoniac and alumine at once. I boil this mixture for a quarter of an hour, at 

 the end of which all the ammoniac is volatilized, provided no more than three or four doci- 

 maftic quintals be operated upon. 1 combine this ammoniac to faturatlon with the ful- 

 phuric acid, and the quantity of fait which I obtain indicates that which was contained in 

 the ammoniacal fulphate of alumine. 



When the pot-afli does not indicate the prefence of ammoniac, which is very feldom, I 

 follow another method to feparatc the fulpliate of pot-afh. I decompofe the alum by 

 means of ammoniac, and, after having waflied the precipitated alumine, I evaporate the 

 liquor to drynefs, and heat the remaining fait in a crucible, till it emits no more while 

 vapours of the fulphate of ammoniac. The remainder is the pure fulphate of pot-afli. By 

 thefe analytical methods, I found that one pound of cryftallizcd alum contains about one 

 ounce 64. grains of fulphate of pot-afli ; but as the alum contains about 0,44 water of cryf- 

 tallization, this raifes the quantity of the fulphate to one ounce 7 gros 17 grains, for a 

 pound of alum, or otlierwife for cryflallized alum, about 0,070, and for dry alum, 0,125. 

 When the alum has been formed with volatile alkali, it is found to contain the fulphate of 

 ammoniac, nearly in the fame proportion as the fulphate of pot-afli. Whence it follows, 

 that a quintal of alum prepared with pot-afli contains, 



1. Sulphate of alumine — — — 49 



2. Sulphate of pot-afli _ »™ — y 



3. Water — _ — _ 44. 



When the alums contain botli the falts here mentioned at once, which frequently enough 

 happens, I ufe lime inftead of pot-afli to difcngagc the ammoniac, and proceed with the 

 refidue as before. 



It may tlicrefore be afcertaincd, by thefe fimple eflays, whether pot-afli or ammoniac or 

 both together have been ufed in a manufaiSlory for the preparation of alum. This proof 



* Hujiis (lixivii magiltralh) caniluro duas aiUlidi dr.nchmas argili.e Colonicnfis in fii!itilcm<.imfniii,itT pul- 

 ".trcin, ct priuci aquae humcftati ; cilorc ebuHitioncm pro. ocavi, qua per decern minula.- coiniiuut.i, ct polU'.T, 

 refrigcrationc peradti, nfidviam fcparavi ar^iilam ; Uitam ficcavi j tandcmquc pmulcraiioDe invtni ^:.;grjna 

 ftiluu, qux aluminit augmcntum 141 gijnorum indicant. 1'crg. ibidem 



Vol. I. — October 1797. T t may 



