MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
57 
Axial Ratios .—The ratios of the morphological axes of any ammonium salt of the 
series are very close to those of the alkali-metal salts of the same group, and usually 
lie within the limits of the latter. 'They thus afford very strong evidence of the 
isomorphism of the ammonium salts with the alkali-metal salts of the group and 
series. 
Cleavage .—The crystals of the whole series of salts show cleavage parallel to the 
orthopinakoid r'{20l}. Four of the ammonium salts, those of the Mg, Ni, Mn, and 
Cu groups, show in addition a good cleavage parallel to the symmetry plane b {010}, 
and in ammonium copper sulphate this cleavage is the more perfect of the two. The 
introduction of the NH 4 radicle in place of an alkali metal thus appears to affect 
the cleavage. A similar effect was observed in the simple rhombic sulphates 
of ammonium and the alkali metals ; but in this case it took the form of the 
suppression in ammonium sulphate of one (b {010}, the more perfect cleavage) of 
the two cleavages (parallel to 6 {010} and c {001}) common to the sulphates of 
K, Rb, and Cs. 
Density and Molecular Volume .—The density of the ammonium salt is the lowest 
in every group. The molecular volume of the ammonium salt, however, is nearly 
identical with that of the rubidium salt of the same group; on the average it is just 
slightly higher, the differences varying from 1‘6 above to 0T6 below the value for the 
rubidium salt. The average difference of molecular volume, for the whole of the 
double sulphate series, between a K and a Rb salt is 9'55 units, between a K and a 
NH 4 salt 10"41 units, and between a K and a Cs salt 22'23 units. The change of 
molecular volume with change of the alkali metal thus follows the order of the atomic 
weights of the alkali metals, and the absolute amounts are a higher function of the 
atomic weight than direct proportionality. The two following tables will render 
these facts clear. 
Relative Densities Compared. 
M 
metal. 
Density 
of 
K salt. 
Difference 
between 
K and Rb 
salts. 
Density 
of 
Rb salt. 
Difference 
between 
K and NH 4 
salts. 
Density 
of 
NH 4 salt.' 
Difference 
between 
Rb and Cs 
salts. 
Density 
of 
Cs salt. 
Difference 
between 
Iv and Cs 
salts. 
Ni 
2-237 
0-349 
2-586 
-0-314 
1-923 
0-286 
2-872 
0-635 
Co 
2-219 
0-348 
2-567 
-0-318 
1-901 
0-277 
2-844 
0-625 
Mn 
— 
— 
2-461 
— 
1-831 
0-279 
2-740 
— 
Cu 
2-233 
0-341 
2-574 
-0-307 
1-926 
0-284 
2-858 
0-625 
Cd 
— 
— 
2-695 
— 
2-061 
0-262 
2 • 957 
-- 
Mg 
2-034 
0-352 
2-386 
-0-311 
1-723 
0-290 
2" 676 
0-642 
Zn 
2-246 
0-345 
2-591 
-0-314 
1 • 932 
0-284 
2-875 
0-629 
Fe 
2-177 
0-341 
2-518 
-0-313 
1-864 
0-278 
2-796 
0-619 
VOL. ccxvi.—A. i 
