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XLVII. Rejections on the pariicular Properfies of Rotmn 

 A urn. By M. Cu raudkau, M^mha- of tlw Soci^tt, of 

 Ftiannacij at Paris , &c. * ' 



M VAuauELiv lias recently analysed tiie different kinds 

 ot alum employed m commerce, and from this analysis has 

 shown that the proportions of the acid and of base ixx'c nearly 

 the same in each kmd. 



M. Hassenfratz observes f, that if Vauquclin finds no 

 sensible difference in the proportions of the acid and bases 

 ot each kmd ot alum, it is however verj' true that dyers 

 allow that Roman alum produces in dveincr, effects which 

 could not be obtamed from the other kinlls. The same 

 chemist ados, that if any other person than Vauquclin had 

 announced this result he should have been inclined to sus- 

 pend his opinion ; but that chemists have assured them- 

 selves ot the exactness of M. Vauquclin, and the confidence 

 to uhich his experiments are entitled. This opinion, ia 

 which I sincerely participate, is still strenfithencd by the 

 labour 1 have been engaged in on alum. 



M. Hassenfratz, however, cannot resist the doubt excited 

 bv the chBerence in the crystallization of Roman alum, 

 which affects the cubic form, and of the other alums the 

 torm ot ^^hlch is octacdral. Jle suspects that this diffe- 

 rence may depend on the tpiantiiv of the base which Roman 

 alum may have greater than the other kinds of alum, 

 though, like them, it may have an excess of acid. 



this observation of M. Hassenfratz on the difference of 

 crystallization ,s very correct; but, as the consequence 

 which he deduces can be only conjecture, it can in no" 

 manner stand in opposition to the analysis ot Vauqueliu. 

 In regard to tlie preference given by the dvers to Roman 

 alum. It IS not the result of an ill founded prejudice: it is 

 certainly true that Roman alum produces in dveincr, effect. 

 which cannot be obtained from the other kinds of alum". 

 I have prepared with Roman alum red morocco as beautiful 

 a. that ot Choisy, win e with any other kind of alum I could 

 obtain otdy disagreeable colours. 



Crystals of Roman alum obtained in a liquor with excess 



^hev i;! r.'";-^' f "''"•?''• '" ^>"''"S the same properties as 

 they had before their solution. 



This experiment, which destroys the conjecture of 

 M llassenlralz in regard fo the excess of base in Roman 



tn?\:^^'T"' "''''/'"' ''^"" ''^'^ ITopcrlies peculiar to it. 

 '>ut nhich cannot be explained by analysis. Unfortunaldy, 



• Frem 4»na'fs dr Cbimii-, No i;^. 

 t Ar.iia'ti lie Cbmie, No. ico. 



there- 



