Note 10 bis — Chapter XXIV · COPPER, SILVER, AND GOLD
George Kamensky translation
Although sulphate of copper usually crystallises with 5H2O, that is, differently to the sulphates of Mg, Fe, and Mn, it is nevertheless perfectly isomorphous with them, as is seen not only in the fact that it gives isomorphous mixtures with them, containing a similar amount of water of crystallisation, but also in the ease with which it forms, like all bases analogous to MgO, double salts, R2Cu(SO4)2,6H2O, where R = K, Rb, Cs, of the monoclinic system.
Salts of this kind, like CuCl2,2KCl,2H2O,PtK2Cy4, &c., present a composition CuX2 if the representation of double salts given in Chapter XXIII., Note 11, be admitted, because they, like Cu(HO)2, contain Cu(X2K)2, where X2 = SO4, i.e. the residue of sulphuric acid, which combines with H2, and is therefore able to replace the H2 by X2 or O. A detailed study of the crystalline forms of these salts, made by Tutton (1893) (see Chapter XIII., Note 1), showed: (1) that 22 investigated salts of the composition R2M(SO4),6H2O, where R = K, Rb, Cs, and M = Mg, Zn, Cd, Mn, Fe, Co, Ni, Cu, present a complete crystallographic resemblance; (2) that in all respects the Rb salts present a transition between the K and Cs salts; (3) that the Cs salts form crystals most easily, and the K salts the most difficultly, and that for the K salts of Cd and Mn it was even impossible to obtain well-formed crystals; (4) that notwithstanding the closeness of their angles, the general appearance (habit) of the potassium compound differs very clearly from the Cs salts, while the Rb salts present a distinct transition in this respect; (5) that the angle of the inclination of one of the axes to the plane of the two other axes showed that in the K salts (angle from 75° to 75° 38′) the inclination is least, in the Cs salts (from 72° 52′ to 73° 50′) greatest, and in the Rb salts (from 73° 57′ to 74° 42′) intermediate between the two; the replacement of Mg … Cu produces but a very small change in this angle; (6) that the other angles and the ratio of the axes of the crystals exhibit a similar variation; and (7) that thus the variation of the form is chiefly determined by the atomic weight of the alkaline metal. As an example we cite the magnitude of the inclination of the axes of R2M(SO4)2,6H2O.
R = K Rb Cs
M = Mg 75° 12′ 74° 1′ 72° 54′
Zn 75° 12′ 74° 7′ 72° 59′
Cd — 74° 7′ 72° 49′
Mn — 73° 3′ 72° 53′
Fe 75° 28′ 74° 16′ 73° 8′
Co 75° 5′ 73° 59′ 72° 52′
Ni 75° 0′ 73° 57′ 72° 58′
Cu 75° 32′ 74° 42′ 73° 50′
This shows clearly (within the limits of possible error, which may be as much as 30′) the almost perfect identity of the independent crystalline forms notwithstanding the difference of the atomic weights of the diatomic elements, M = Mg, Cu.