31^ Analjifis tj the Arfeniaies 



to retain a much greater quantity. This, therefore, is a fiif- 

 ficient proof of the inaccuracy of attempting to precipitate 

 copper from its folutions, by an alkahne carbonate ; for car- 

 bonate of focia has, like carbonate of potafli, the property of 

 dilVohing and cryftallizing, as a triple fait, with hydrate of 

 copper. 



In addition to this fource of error, we may add the very 

 uncertain operation of one metal upon the faline combination 

 of another, whatever may be their affinity for oxygen. In- 

 deed I have fo often remarked this very great uncertainty, 

 that I was pleafcd to find the obfervation had occurred to 

 Mr, Prouft; and I have conflahtly found that more depend- 

 ence can be placed upon the method I have adopted, than 

 upon any attempts to precipitate the metals by each other. 



With regard to efficacy and promptnefs, tin or zinc is pre- 

 ferable 10 iron J but, with any of the three metals, a phasno- 

 menon occurs, for which I have not been able to account 

 in anv manner the lead fatisfa£lorv. As the effects are more 

 ftriking with zinc than with the other metals, I fhall enter 

 into particulars refpc&ing the ufe of that metal onlv. 



When a bit of zinc (or tin, or iron,) is immerfed in a fo- 

 lution of muriate of copper, a precipitation begins, and all 

 the copper is finallv feparated in a few hours*: but, if a 

 little muriatic acid is added to this folution, and the zinc 

 then immerfed, a violent eftervefcence takes place; hydrogen 

 gas is difengaged, and, in lefs than two minutes, the copper 

 is lo completely precipitated in the metallic ftate, that neither 

 ammonia, nor even fulphuraled hydrogen, can difcover any 

 veitige of its prefence. 



It would appear from this, that hydrogen is, in this cafe, 

 the real reductive and precipitant of the copper. Yet, how 

 can we reconcile the evident contradiiSlion, thai, in one cafe, 

 zinc with muriatic acid will dccompofe water, that is, that 

 zinc and muriatic acid will attract oxygen more ftrongly than 

 hydrogen can, yet that, in the other cafe, hydrogen, whofe 

 affinity for oxygen is weaker than that of zinc and muriatic 

 acid, will be more fpeedy and powerful than zinc and mu- 

 riatic acid, in atlrafting that oxvo;en from copper ? Again, 

 how is it polfible that zinc and muriatic acid will, in pre- 

 ference to taking oxvgen immediately from the oxide of cop- 

 per, dccompofe w ater, the hydrogen of which will unite with 

 the oxv'icn of the oxide of copper, again to become water, 

 which it originally was? All this appe;;rs to me, I confefs, 

 as contradictory as to fay, one is lefs than two, two are lels 



* If any quantity of neutral fait is prcftnt in the folution, the precipi- 

 tation is much retarded, and is teldcni corrpkti-, 



than 



