MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
15 
the prism is relatively longer than is shown in the two figures. In the great majority 
of crops prepared the type was that of fig. 4, or of the same elongated vertically, the 
faces of the basal plane c {001} predominating at the terminations. But in several of 
the crops thee faces were relatively narrower and the faces of q {011} proportionately 
larger, as shown in fig. 2. In all the crops the prism faces p {110} were always 
important, and indeed usually predominated. The two faces of the clinopinakoid 
b {010} were often well developed, although many crops showed these faces only as 
strips, and not uncommonly they were either entirely absent or mere lines. The 
faces of the orthopinakoid r' {201} were usually very subordinate, as shown in 
the figure. The two hemipyramids o{lll} and o'{111} were usually represented 
by some of their faces, and the latter form often by all its faces, although 
small. The orthopinakoid a {100} was only very rarely developed to a measurable 
extent. 
The next three tables compare the morphological constants of the four salts 
containing cobalt. The first of these shows that the axial angle /3 of ammonium cobalt 
sulphate is very close to that of caesium cobalt sulphate, and that the axial ratios of 
the ammonium salt are very clearly such as place that salt in the same isomorphous 
series as the three cobalt salts containing the alkali metals. 
The angular comparisons instituted in the second table are supplemented by 
a further short table showing the average and maximum changes of angle which occur 
when potassium is replaced by the other three alkali bases. These changes are seen 
to be nearly the same for the ammonium and caesium replacements, and these again 
are twice as great as the average and maximum changes of angle when rubidium is 
introduced instead of potassium. A critical analysis of the comparative table of angles 
also shows that of the 36 angles compared 32 show changes, when ammonium replaces 
potassium, in the same direction as when the latter is replaced by the other two 
alkali metals, and the only four exceptions are in cases where the changes are very 
minute.' Of the 32, 29 show larger changes than for the rubidium replacement, and 
11 show even larger changes than for the caesium replacement. 
Comparison of the Axial Angles and Axial Ratios. 
Axial angle. 
Axial ratios. 
A 
a :b: c, 
Potassium cobalt sulphate . 
104° 55' 
0-7404 : 1 : 0-5037 
Rubidium ,, ,, . . 
106° V 
0-7391 : 1 :0*5011 
Ammonium ., ,, , 
107° 2' 
0-7386 : 1 :0’4975 
Caesium ,, . 
107° 8' 
0-7270: 1 -.0-4968 
