sr6 



Mistake of 

 Bergman. 



Potash neces- 

 sary. 



« 



Vauquelin. 



Alum a con- 

 stant salt, 



but frequently 

 contaminated 

 with ammonia 

 and iron. 



Preference 

 givfen to Ro- 

 man alum by 

 the dyers sup- 

 posed to be 

 without cause, 



COMPARISON OF DIFFERENT KINDS OF ALUM. 



of some cities in Syria. In the 15th century it was brought 

 into Europe, and soon becfime common in Ituly, where that 

 of Tolfa required great reputation by the constant unifor- 

 mity of its product, as well as its purity. But this art, slill 

 m its infancy, was very slowly improved; and it was not till 

 three hundred years after, when chemistry was sufficiently 

 advanced to discover the intimate nature of substances, that 

 it made some progress. Mar^ratf, Monnet, Erxleben, and 

 Bergman, then analysed all the kinds of alum most gene- 

 rally known. Bergman in particular was so well aware of 

 the importance of the question, that he wrote a dissertation 

 of considerable length on the history, preparation, analysis 

 and purification of alum ; in which he lays particular stress 

 on the' necessity of carefully separating the iron from it by 

 repeated crystallizations, by means of^which he says he ma- 

 nufactured alum even purer than that of Rome. He had 

 some erroneous ideas however, which modern chemists have 

 corrected. 



Mr. Chaptal first perceived Bergman's mistake in pro- 

 posing to saturate the acidulous solutions with clay ; and the 

 simultaneous discoveries of Decroissilles, ChaptaV, and Van* 

 quelin, on the action of potash in the formation of alum, 

 and on the various combinations of the sulphuric acid with 

 alumine, left us nothing more to wish on these heads. 



The knoVvledge thus acquired gave rise to several alum 

 works, the produce of which, though approaching that of 

 Tolfa, was not able to diminish the preference given it by 

 all manufacturers, or to lower the price it bore. The learned 

 awaited with impatience the solution of this important pro- 

 blem, when Mr. Vauquelin made known the result of his 

 analyses of Roman alum' compared with that of some other 

 kinds mo?t generally known. He showed, that the p'ropor- 

 tion of the constituent, principles of alum is always the 

 same, and that they differ only in consequence of a few par- 

 ticles of sulphate of ammonia and of iron, which he could 

 not find in any appreciable quantity in Roman alum. He 

 concluded his interesting analysis by saying, that, if there 

 w^re so much difference in alum as the dyers say, chemistry 

 in its present state was not able to detect the cause ; but that 

 it appeared to him more natural to suspect them of exagge- 

 ration : 



