or THE DOUBLE SELENATES OF THE SERIES RoM(Se 04 ). 2 , 6 H 20 . 
273 
Circle Readings for production of Uniaxial Figure. 
Section 1. Section 2. Section 3. Mean reading. 
In air.4° 59' 5° 27' 5° 16' 5° 14' 
In cedar oil ... 5 23 5 27 5 18 5 23 
Mean of two series.5 19 
The reading 5° 19' corresponds to the passage of blue light of wave-length 466 
throuo'h the exit slit of the monochromatic illuminator. 
o 
Dispersion of the Median Lines. —Although there is such large dispersion of the 
optic axes, the median lines remain fairly constant. The first median line lies nearer 
to the axis a for red C-light than for greenish-lfiue F-light by about 15'. 
Effect of Rise of Temperature on the Optic Axial Anrjle .—Sections 1 and 3 were 
studied at temperatures up to 97°. The phenomena presented were highly interest¬ 
ing, for within this comparatively small range of temperature (the most that can be 
employed as regards the upward direction on account of the presence of water of 
crystallisation), the uniaxial cross and circles are produced for all wave-lengths of 
light in turn, from 466 in the blue to the extreme red of the spectrum. The axes 
are observed to begin to approach each other as soon as tlie temperature commences 
to rise appreciably. 
The following table represents the temperatures (corrected for the slight conduc¬ 
tion of the crystal holder as descril;ed in the memoir concerning emsium selenate, 
‘ Journ. Chem. Soc., Trans.,’ 1897, 895) at which the uniaxial figure is produced for 
different wave-lengths of light :— 
Mean Corrected Temperatures for Production of Cross. 
For F-light at 34° 
„ T1 „ 60 
„ Na „ 78 
„ C „ 91 
„ Li „ 94 
When the cross is produced for tliallium light at 60° the axes are still separated 
13° 0' for sodium light and 20° 30' for lithium light; and when tlie temperature 
attains 78° and the uniaxial figure is formed for sodium light, the axes remain 
separated \ 3° 30' for lithium light. 
Repeated heating of the same section would appear to slightly lower ])ermanently 
the temperatures at which the cross is produced. 
The second series of six figures in the Plate represent the phenomena oliserved at 
78°, in Li-, C-, Na-, T1-, and F-light, and m light of wave-length 466. 
2 N 
VOL. CXCVII.—A. 
