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Panfication of Zinc. 359 



fulpliiirated hydrogen. When it no longer becomes coloured, a greater quantity of hydro- 

 fulphurated water mud be added to it, when the zinc will in its turn fall down of a light 

 yellow colour. If there had been iron, cobalt, nickel, or manganefe in the folution none of 

 them would have been precipitated by this re-agent. 



From the preceding experiments on zinc, M. Prouft draws the following conclufions. 

 ift. In whatever acid this metal be diflblved, it conftantly abforbs the fame quantity of oxy- 

 gen. 2d. If it contain metals capable of fuperoxydation, they pafs to this ftate when the 

 folution has been efFetTled either by nitric or oxygenated muriatic acid. 3. In a muriatic 

 or fulphuric folution this metal is as much oxided as it can be, but iron on the contrary 

 is only at its minimum ; and this is the reafon why this folution does not become coloured 

 with gallic acid, unlefs it has been expofed to the air. 4th. To demonftrate the prefence of 

 iron fome drops of oxygenated muriatic acid muft be added, or it may be boiled with a fmall 

 quantity of nitrous acid. 5. The carbonate, which is very white whilft under water, be- 

 comes yellow as foon as it is brought into the air, becaufe the iron pafTes quickly to the 

 maximum of oxidation. 6th. Sulphate of zinc, which has had time to abfoib in the at- 

 mofphere as much oxygen as is neceflary to elevate the iron to its maximum, immediately 

 affords yellow carbonate of zinc. 7. Zinc, which is purified by diftillation, or by the 

 following humid procefs, is alone proper to afford the true zinc- white for painting. 



Repeated criftallization of the fulphate of zinc, and immerfion of plates of this'metal 

 into its folution appeared to M. Prouft, to be very infufiicient means to feparate the foreign 

 metals. To accomplifli this objeft, 30.57 grams (one Fr. ounce) of nitric acid are put into 

 about 2 pounds of the faturated folution of fulphate of zinc, and the mixture flightly boiled. 

 Pot-alh is then mixed with it to faturate the excefs of acid, and to precipitate nearly 

 15.28 grams (3 or 4 gros) of the matter ; this mixture is boiled again, and the precipitate 

 is foon obferved to pafs from white to yellow. If after fome minutes of ebulition fomc 

 white particles are remarked amongft the yellow depofit, it is a certain fign that not an atom 

 of iron remains in the folution of zinc. But if by this method the iron has been entirely 

 feparated from the zinc, manganefe may yet be prefent, if any portion exifted in the ore ;, 

 and M. Prouft has frequently met with it. 



To clear it of this new oxide the fulphate of zinc is dlffolved in boiling water, and preci- 

 pitated by carbonate of pot-afli, fo as ftill to leave a fmall quantity of oxide of zinc in folution. 

 The depoCt is kept feveral days in the liquor, in order that the oxide of manganefe, which 

 has been precipitated, being more ftrongly attrafted by the acid than the oxide of zinc, may 

 precipitate that portion of the latter which had been left in the fluid, and become diffolved 

 in its place. Then the fulphate of zinc affords an oxide of the moft perfedl whitenefs which 

 may be employed, very advantageoufly, for painting. 



Thefe are the fads contained in M. ProuR's Memoir. They are numerous and Interefl:- 

 ing. Some of them are new to us; the moft part, though already known, are prefented 

 under new relations and rendered applicable to the arts. The experiments, by the help of 

 which he has difcovered thefe fads, are ingenious. His explanations, though concife,. 



appear 

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