MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
47 
density determinations the crystals have had to be freshly prepared and used very 
soon after removal from the mother liquor. Their goniometrical measurement, 
however, has offered no difficulty. 
The relative development of the faces is frequently so different from the main types 
illustrated in figs. 11 and 12, that it is not always easy to recognise the various faces 
immediately. The ten small crystals measured were selected from the two best crops 
of a very large number prepared. They afforded very satisfactory goniometrical 
results (see accompanying table of angles) in all cases but those involving the faces of 
(/{ill} in the zone [_cpo , ~\ ; for when this zone was adjusted these faces invariably 
showed multiple images of the signal, due to more or less striation, so that the mean 
measured values of the angles po' and o'c differed rather more than usual from the 
calculated values. 
The rarer forms common to this monoclinic series of double salts are exceptionally 
well developed on the crystals of cadmium ammonium sulphate ; for instance, the 
faces of a {100}, o {ill}, p'{l20}, and j»3 /// { 130} have been frequently observed quite 
large. Moreover, single faces of the commoner forms b {010} and r'{20l} are 
occasionally found phenomenally developed; thus several crystals were found 
roughly hemispherical, the large flat base being a particularly large and perfectly 
plane b- face, while others were found with a similarly predominatingly developed 
A-face. 
The morphological constants of the three cadmium-containing salts are compared in 
the next two tables. As already explained, potassium cadmium sulphate with 6H 2 0 
has so far resisted all attempts at preparation. The first table shows that the axial 
ratios of the ammonium cadmium salt are very close to those of the rubidium cadmium 
salt. The axial angle ft of the ammonium salt is not far removed from that of the 
caesium salt, but is not so close as in all the other groups investigated. The morpho¬ 
logical angles of the ammonium salt are shown in the second table to be related to 
those of the rubidium and caesium salts in a very similar manner to what has been 
shown to occur in the other groups, for which it has been possible to effect a complete 
comparison and to determine the differences in angle of all three salts from the 
potassium salt. They indicate conclusively that the ammonium salt belongs to the 
same isomorphous series as the rubidium and caesium salts containing cadmium. 
Comparison of the Axial Angles and Axial Ratios. 
Axial angle. 
Axial ratios. 
Rubidium cadmium sulphate . 
Ammonium „ ,, 
Csesium ,, ,, 
ft. 
105° 53' 
106° 41' 
107° 11' 
a : b : - c 
0-7346 : 1 : 0-4931 
0-7364: 1 : 0‘4931 
0-7259 : 1 : 0-4906 
