JMUTUAL FRECIPITATIONS OF MEtALLIC OXIDES. 271 



of Iron precipitates the red oxide, and that it is, con fequently. Black oxide of 

 very eafy to have green folutions of' iron without red oxide. ^^"" precipitatea 



Into a ("olution of fulphate of zinc of commerce, which is Experiments 

 known to contain much iron, I Doured a little potafh, to^^'^^^^^^^'p'^^^*^ 

 produce a precipitate, and 1 agitated and heated the mixture, merce. 

 On examining the precipitate I found oxide of zinc and a 

 little iron very much oxided, and neverthelefs the liquor ftill 

 contained much iron, but it was at the minimum of oxidation : 

 an addition of alkali only leparated oxide of zinc. I divided 

 the filtered liquor into two portions ; into one I poured oxige- 

 nated muriatic acid, and I boiled the other with a little nitric 

 acid. Potafli then poured into the two liquors feparated all the 

 iron from them, fo that there only remained a very pure fuiphate 

 of zinc, containing only a little fulphate of potafli, which it is 

 eafy to avoid by employing oxide of zinc, recently prepared 

 and well vvaflied, to feparaie the iron. On making the fame Oxide of ^inc 

 experiments on oiher folutions of zinc, Iconftantly found that P'T^'f^'^/f" '■'^'^ 



• 1 ,- ■ - ■ 1 1 • i' r • 11 , oxide of iron, 



oxide or zinc precipitated red oxide ot non, and that, on the but ,s p.edpi- 

 contrary, it was precipitated by the black oxide. t'^ted by the 



A folution of zinc in vjitric acid may be directly obtained, 

 fufficiently pure, by di^ToIving it very rapidly : great part of 

 the very oxided oxide of iron is precipitated ; and that which 

 remains in folution requiring a great excefs of acid, is precipi- 

 tated by diffoiving another q lantify of zinc. But if the folution 

 has been made (lowly, it retains much iron, which being but iittle 

 oxided, is retained very ftrongly. 



It is well known that, in all experiments of this kind, the The quantity of 

 precipitate may be compofed of one or of two oxides, accord- '^'^^';^^J^P[^>^^lJ^ 

 ing to the quantity of alkali employed ; but the better fo 

 obferve what pafies, it is advifeable to put very little alkali into 

 the metallic folution, the precipitate being in that cafe compofed 

 ©f only one oxide. 



By continuing to follow the fame procefles I found, that when Importance of 

 the iron is very much oxided it is precipitated by oxide of cop- the mutual pre» 

 per, and that the inverfe takes place when it is but httle fo. ^Jjpp^rand Irox^ 

 Here are two very important confequences, becaufe they may 

 be very frequently applicable in the arts : the firlt is, that all 

 the iron may be feparated from a folution of copper; the fecond, 

 that all the copper contained in a green folution of iron may be 

 abltraaud. 



Several 



