Mutual precipitations op metallic oxides. 275 



heve that this caufe can have much efFecl, it being ftrongly 

 counteraded by the acidity which ahnoft all the metallic folu- 

 tions pofTefs, and by the infolubility of the oxides. Thus, 

 although it appears to me that iron, when little oxided, has 

 more affinity for muriatic acid than when it is greatly oxided, 

 I fliould rather attribute the precipitation of the latter by the 

 former to the great excefs of acid which its folution requires, 

 than to its weaker affinity. 



Neither, for the fame reafons, do I believe that the affinity but is not the 

 of the different oxides for the acids, an affinity which I mea- ""^^ '^^" ^'^^'.^ 

 fure, with Cit. Berthollet, by the capacities for faturation,ut'iom/'^"'^" 

 can be conildered as the caufe of their mutual precipitations. 



Befides, there is one conilderation of fome importance which Influence of the 

 fliQuld be taken into the explanation of the mutual precipita-"'*'"^'^ ^^"^' 

 tions of the oxides ; it is that, in a cafe where the precipita- 

 tion of a metallic folution is produced by means of an alkali, 

 the precipitate retains fome of the acid which can favour its 

 folution ; fo that an oxide which could retain much of the •- 



acid, would diflblve more readily than that which could only 

 retain lefs. It mud really be fo with iron, which, when it is 

 precipitated from a green folution, retains much more acid than 

 when it is precipitated from a red folution, and which diflolves 

 much better in the acids in the firft cafe than in the fecond. 

 This more ready folution cannot, however, be confidered as 

 a caufe of the mutual precipitations of the metallic oxides ; 

 it may be very favourable to them, but cannot determine them. 

 In fa6l, we fee that the oxide of copper, which retains much 

 ^cid, is neverthelefs precipitated by oxide of filver, which 

 does not fenfibly retain any, when they are precipitated from 

 their nitric folutions by potafn. 



CONCLUSION. 



THE metallic oxides are mutually precipitated from their General infer- 

 folutions. Several caafes may contribute to this; but among ^""^* 

 the number of the principal muft be placed the property which 

 they have of neutralizing the acids unequally. 



This property has furnithed us with the means, 1/. Of 

 freeing a green folution of iron from the red oxide which it 

 may contain ; 2d. Of feparating the fulphate of zinc and tliat 

 of copper from the iron which is always found in them ; 3d. 

 ,Of having a green fulphate of iron free from copper; 4//*. Of 

 T 2 readily 



