of the Metallic Oxides. 101 



carbonation, like dehydration, does not occur all at once, but 

 is continuous and progressive according to the temperature. 

 The course of this decomposition is clearly shown in parti- 

 cular cases, notably in the decomposition of certain insoluble 

 carbonates in the presence of water, and at a slightly elevated 

 temperature, and often at the moment of precipitation. The 

 decomposition results in the formation of basic carbonates, 

 anhydrides or hydrates, oxy- or hydroxy-carbonates, of a com- 

 position more or less complex, and in the smallest molecule 

 of which are accumulated several molecules of metallic oxide or 

 hydroxide. A large number of metals, e.g. Mg, Zn, Pb, Cu, 

 Bi, Hg, Al, Cr, Fe, Co, Ni, &c, furnish remarkable examples 

 of this general fact. These insoluble basic carbonates are 

 usually obtained by precipitation with soluble carbonates, 

 either from hot or cold metallic solutions. The composition 

 of these precipitates for the same metal, frequently varies 

 between more or less wide limits, depending on the special 

 circumstances under which the precipitation has taken place. 

 It is evident that these various compounds cannot all be con- 

 sidered as well defined. There are some however, the che- 

 mical individuality of which has been well established, and a 

 number of these, either natural or artificial, are capable of 

 assuming the crystalline state. The following are examples : — 



a. Mg . . . 3C0 2 .4MgO-f4H 2 0. 



4C0 2 . 5MgO + 5H 2 0. Magnesia alba. 



b. Zn. Composition very varied. 



ZnC0 3 .Zn(OH) 2 + 2H 2 0. 

 2ZnC0 3 .Zn(OH) 2 . 

 2ZnC0 3 . 3Zn(OH) 2 . Crystalline. 



4ZnC0 3 . 7Zn(OH) 2 + H 2 (the preceding dried at 100°). 

 3ZnC0 3 .5Zn(OH) 2 + H 2 0. 



ZnC0 3 . 2Zn(OH) 2 + H 2 (natural) . 



ZnC0 3 .3ZnO + 2H 2 0. 



ZnC0 3 .7ZnO + 2H 2 0. 



c. Pb. The following, according to H. Rose, represent the 

 composition of the hydrocarbonates of lead formed under 

 definite conditions : — 



!by mixing cold and concen- 

 trated solutions of sodium 

 carbonate and lead nitrate in 

 equivalent quantities. 

 5PbC0 3 .Pb(OH) 2 . . . ditto, but dilute and cold. 

 3PbC0 3 . Pb(OH) 2 . . . ditto, but dilute and boiling. 

 2PbC0 3 . Pb(OH) 2 . . . ditto, but with excess of alka- 

 line carbonate. 



