50 
DR. A. E. H. TUTTON ON THE 
tions of this salt by the pyknometer method, owing to rapid efflorescence, which was 
greatly accelerated by powdering and necessitated rapid working and weighing while 
covered with carbon tetrachloride, the pyknometer liquid. The immersion method is 
free from the powdering difficulty, and it is only necessary to use freshly grown 
crystals, and to preserve them under benzene until the moment of dropping them 
into the immersion liquid mixture of methylene iodide and benzene. This was done 
in the cases of the five determinations now recorded, and the result, 2‘695, is much 
higher than the former pyknometer one, and can be accepted with confidence. 
Volume Constants of the Cadmium Group. 
o i, I Molecular 1 
weight. 
Specific 
gravity. 
Molecular 
volume. 
Topic axial ratios. 
RbCd sulphate . 
579-36 
2-695 
214-98 
X : I ■ w 
6-2539 : 8'5133 : 4'1980 
NIRCd 
445-42 
2-061 
216-12 
6-2838 : 8-5332 : 4'2078 
CsCd 
673-36 
2 • 957 
227-72 
6-3497 : 8-7472 : 4'2914 
The comparative table of the volume constants shows that the ammonium cadmium 
salt occupies a position just beyond that of the rubidium cadmium salt, the molecular 
volume and all three topic axial ratios being slightly higher. 
Optics. 
No optical investigation of this salt appears to have been hitherto undertaken. It 
is a matter of some difficulty, as ground surfaces of the crystals effloresce with great 
rapidity; also the use of warm wax on the crystal holder for cementing purposes is 
precluded, as rise of temperature provokes instant opacity from loss of water of 
crystallisation. By careful rapid work, however, the use of a special wax, and 
immediately covering ground surfaces with very thin truly plane glass plates cemented 
by hard balsam in benzene, all these difficulties were eventually overcome. 
Orientation of Optical Ellipsoid- -The optic axes lie in the symmetry plane b {010} 
and the sign of the double refraction is positive. The first median line corresponds to 
the axis y of the optical indicatrix and the second median line to a. 
A pair of section-plates parallel to the symmetry plane afforded the extinction 
angles:—- 
Inclination q/2M.L. from normal to c {001}. 
Plate 1 ... 5° 8' Plate 2 ... 5° 20' 
Mean . . . 5° 14'. 
