in the ProduBion of Alum. St 



In 6rder to difcover the proportion of the alumlne to the fulphuric acid, nothing more is 

 required, tlian to combine as much of the earth with the acid, as it will dilTolve in the 

 common temperature (from 50° to 60° Fahrenheit). An excefs of the acid will, in that 

 cafe always remain, as (hewn by the teft of Litmus. For at that temperature the acid does 

 not take up fo much of the argil as to become neutralized. The folution muft, however, 

 be left {landing long enough (24 hours) upon the remaining undiflblved part of the earth, 

 becaufe the argil is diflblved much more flowly by this acid, than the calcareous and mag- 

 neflan earths. 



In order to difcover the due proportion of pot-a(h in this procefs, Marggraf dirccSls us 

 to continue adding gradually of the alkaline lye by drops, till a light pulverulent precipitate 

 (that is to fay, precipitated argil) appears. In the following experiments I added fo much 

 alkali, whether pot-afli, or either of the other two, that (the quantity of diflblved pure 

 alumine being half an ounce) there remained a fufficiently perceptible quantity of preci« 

 pitated earth, though very fmall in proportion to the wliole. For, it is to be obferved, 

 that the very firft drops of the alkaline lixivium, if neither itfelf nor the folution of the 

 earth be too weak, produce a turbidnefs ; but this firft precipitate is re-diflblved in the ex- 

 cefs of the acid prefent. I am much inclined to fuppofe, that it may be immaterial to 

 determine the precife quantity of alkali, becaufe I have conftantly obtained alum by the 

 addition of pot-a(h, notwithftanding that in fome inftances I added more of it, and in 

 others lefs. 



To afcertain the regular form of the cryftals of alufn, one or more threads fliould be 

 hung into the lye, in order that perfect, or nearly perfeft octahedrons may be formed 

 round them ; for thofc cryftals which are formed at the bottom, or on the fides of the 

 velTel, are always defective in one or more of their angles. 



Having decided from thefe experiments, that a fmall portion of alkali is eflentially re- 

 quifite to the production of fulphate of alumlne, I mentioned the fadt in the advertifement 

 of Prof. Lampadiuis Chemical EJfays. It cannot be denied, but that in one of his experi- 

 ments * he has obtained alum, from mere argil and fulphuric acid, without the addition of 

 either pot-afli or urine ; but not to mention, that the quantity of alum obtained by the faid 

 pfocefs was indeed exceeding fmalli in proportion to what has been produced on adding 

 pot-afti or urine, it is probable, tlfat as much vegetable alkali as was fufficient for the for- 

 mation of that fmall portion of alum may have entered into the crude lixivium, from the 

 afhes of the wood employed in the roaftlng of the aluminous ore, and of which a greater or 

 lefs portion is unavoidably mixed, with the roafted mineral. Or that alkali may adtualLy 

 have pre- exifted in the ore ; (or if not, at leaft the nitrogen and hydrogen, from which the 

 alkali was generated, may have been prefent.) Profejf. Klaproth f. has difcovered pot-afli in 



• La?npadius, loc, cit. pag. 16. 



■\ Klaproth difcovered in the natii>e alum from M'ifeno fo l^rge a proportion of pot-afti, that 1000 grains 

 of it afforded 470 grains of cryftallizeil ahim, and in thefe 27 grains of fulphate of pot-afti. (Beytrage 

 xur Ketttttifs der Mineral Koerper. I.Berlin, 1795. deile 313.) 



H 2 the 



