£0 On the difftnnt EffeBs of the Alkalis^ 



the addition of pot-afh or urine unneceffary, and with this view he had, in the alum-works 

 of Lederbur at Weifgrun, added to the crude lye fom^ white argil, dug up near the village 

 Chotta. 



The fuperabundant acid here fpoken of, is not that portion which is eflential to the alum, 

 the acid in this fait being never fully faturated with argil; but that quantity only is meant, 

 which may happen to be prefent in the lye as foreign, or not belonging to the compofition 

 of alum, as is the cafe, when the aluminous ore is too abundant in fulphur, or when a 

 fufhcient portion of its argillaceous part has not been combined with the fulphuric acid 

 generated in the procefs. 



If we admit that the crude lixivium of alum docs really contain an excefs of acid, afld 

 that it is this only which impedes the cryftallization of the fait, the addition of argil feems 

 much more proper, than that of either pot-afli or urine; bccaufe more alum muft Be ob- 

 tained Iby the former, while the latter affords fulphate of pot-afli or ammoniac, which 

 diminifli the purity of the produdl. 



But Marggraf* has long ago obferved, that the fulphate of alumine cannot be generated 

 from argil and fulphuric acid alone, and that an alkaline fait is one of its effential ingre- 

 dients. To prove this faft, I have feveral times diffolved fome aluminous earth (that had 

 been precipitated from Roman alum by means of pot-afh, and properly edulcorated) in 

 fulphuric acid carefully reflified, and attempted to cryftallize the clear filtered lye, by 

 evaporation and cooling. But I never obtained the lead appearance of cryflallized alum. 

 The mixture confifting of argil and fulphuric acid, rather in excefs, is more foluble in 

 water than true alum : fo that two parts of water at a temperature of 50° Fahrenheit, are 

 fufficient to diflblve one part of that mafs, merely dried without ignition; whereas the 

 alum requires at the fame temperature, 35, or more parts of water, to diffolve it. Even 

 after that lye had been reduced by evaporation to a much fmaller compafs, than a lixivium 

 of true alum would admit of without cryflallizing, it continued liquid, though expofed to 

 a freezing cold. On evaporating it ftill farther, and by fubfequent cooling, it affords in- 

 deed a compaft mafs, but without any determinate figure, which occupies the bottom of 

 the veffel in the form of a dried jelly. It was but feldom that, in a certain degree of the 

 evaporation, I could perceive a kind of cryftallization in groups; but there was no ap- 

 pearance of fuch hard o£lahedral cryftals, as prefent themfelves in a folution of true alum, 

 containing upwards of twenty parts of water to one of fait diffolved in it f. 



If, on the contrary, to the above folution of argil in fulphuric acid, a fmall quantity of 

 vegetable alkali only be added, and it be then evaporated and left to cool, regular cryftals 

 of alum will be obtained at each fubfequent refrigeration. 



• Marggraf, Cliim, Schriften. I. Berlin, 176S. S. I96. 



+ Marggraf, however, obferved that when the argil was ftrongly calcined, it afforded fome cryftali 

 with the fulphuric acid, which refembled thofe of alum, but were not fo fine as thofe obtained by the ad- 

 dition of an alkaline lye. I wi(h he had ftated this more accurately. But, as my prefent object was to 

 afcertain, which of the alkalis may be inoft fuitable to the generation of alum, I have not yet repeated, 

 that expeciment. 



