tOMPAnrSON OF nrFFERENT KINDS OF ALUM. 



289 



Wool loith sulphate of ammonia. 



Exp. 59 Weld. 



59. .. . . .Cocliiaeal. 



Silk with aliun having an ammo- 

 iiiacal base, 



Exo. 60 Weld. 



Such are the facts we were dcs'iroiis of laying before the General dqK 

 Institute, relative to the long' undecided question of the au- ^"cuons, 

 periority of Roman over all other alums. They afford us 

 an exact and complete coincidence between the results of 

 our analyses and of our experiments with dyes : they show 

 us, that much too extensive an action has been ascribed to 

 the sulphiite of iron, the wholfe of the influence of which 

 we have pointed out, at the snine time marking its limits: 

 and in particular they pvove, that the opinion of the evclu-« 

 sive advantages of Roman alum, formerly perlsaps sufficient- 

 ly just, is now to be considered as an errour successfully 

 combated by theory, and demonstrated by experiment. 

 These facts lead us directly to the following consequences. 



1. All alums contain precisely the same proportions of sul- Alum Itself 

 phuric acid, alumine, potash, and water; though they pro- ^^®"^^'^^^- 

 duce sensible differences with reagents, and in their applica- 

 tion to the art of dyeing. 



'•1. These diiferences arise solely from the unequal quanti- D'ffers from 

 ties of sulphate of iron found in them, amounting merely to beuig conta- 

 a few thousandth parts, for they disappear completely on ir^^'^ 

 the purification of the alums, and are reproduced with the 

 same intensity, if we. refetore to them as much sulphate of 

 iron as had been abstracted. 



3. The Roman alum contains the least sulphate of iron : Roman alum 

 the alums of Bouvier and Curaudau aflbrd us a little more, f'-'^t^rom it^ 

 but the quantity they contain is appreciable oidy by reagents, 



and on silk in weld and cochineal colours. In the alums of 

 Javelle and Liege prussiate of potash immediately indicates 

 the presence of sulphate of iron. 



4. Roman alum does not merit the exclusive preference Other alum 

 given it over other alums, for we have oUtained on wool, ™'*y ^^ {"^de 

 cotton, and silk, with Liege alum purified by uieans of wa- 

 ter, and even with the alums of Bouvier and Curaudau, us 

 fine and brilliant colours as those produced by Roman alum : 

 and if the latter appeared to us to have tlie advantage over 



Vol, XVTII— Dec. I8O7. U the 



Qquul to it. 



