MONOCLINIC DOUBLE SULPHATES CONTAINING AMMONIUM. 
55 
Dispersion of the Median Lines .—The inclined dispersion is very minute. From 
observations in toluene with sections 1 and 2 it was ascertained that the first median 
line lies 8' nearer to the axis a for Li red light than for greenish-blue F light. 
In the next table the optic axial angles of the three cadmium salts are compared. 
Optic Axial Angles 2V a of the Cadmium Group. 
AmCd sulphate. 
RbCd sulphate. 
CsCd sulphate. 
Li. 
o / 
72 45 
72 20 
o / 
68 4 
C. 
72 46 
72 21 
68 2 
Na . 
72 51 
72 26 
67 53 
Ti ...... . 
72 56 
72 31 
67 44 
F. 
73 1 
72 37 
67 28 
It will be observed that the optic axial angle of the ammonium cadmium salt is 
only very slightly larger than that of the rubidium cadmium salt. 
Summary of Conclusions. 
The results of this investigation of the five remaining ammonium salts of the double 
sulphate series are perfectly in line with and fully confirm those already derived, in 
two former communications, from a study of three other ammonium salts of the series. 
The chief conclusion is that the close similarity of the crystal angles, the axial ratios, 
the volume constants, and the optical constants of these ammonium salts clearly 
entitles them to places in the monoclinic isomorphous series iLM(S0 4 ) 2 . 6FLO, It 
being represented by the NH 4 radicle; but that they are not so definitely related, 
“ eutropic ” as it has been termed, as are those salts of the series in which II is 
potassium, rubidium, or caesium. Now interchange of these Pi,-metals has been shown 
to exert a comparatively great and dominating effect on the properties of the crystals, 
progressively with their atomic weights, while change of the M-metal only produces 
slight change. But this law of progression of the crystallographic properties, both 
morphological and physical, with the atomic weight of the alkali metal It, which 
law is the essence of “ eutropism,” cannot apply as regards ammonium ; for we 
are here dealing with a radicle group of elements, NH 4 , and not with three simple 
interchangeable elements of the same family group of Mendeleeef’s table, 
definitely progressive in atomic weight and in all the properties which accompany 
atomic weight. Yet the group NH 4 possesses the singular power of chemically 
replacing these alkali metals in their salts, as also does the metal thallium, which 
is well known to be of a different nature to the three alkali metals potassium, 
rubidium, and csesium. It has now been shown that the replacement occurs in these 
