492 MR. A. E. TUTTON ON THE THERMAL DEFORMATION OF THE CRYSTALLISED 
salts, which is the axis c in both the potassium and caesium salts and the axis a in the 
case of the rubidium salt. In order to follow the parallelism further, it wiU be 
necessary to compare the linear coefficients of expansion for higher temperatures, the 
values for 50° and 100° sufficing for the purpose. In the following table are given 
the values of a = a + 2bt for 0°, 50° and 100°. The directions are also indicated of 
the axes of the optical indicatrix, namely, the first median line, the second median 
line, and the intermediate axis of the optical ellijDSoid. The sign of the double 
refraction of the crystals is also given, as this determines whether the first median 
line is the maximum or the minimum axis of the optical indicatrix, the former being 
the case for positive double refraction and the latter for negative. 
Comparison of Linear Expansions at Different Temperatures with the 
Optical Indicatrix. 
Salt. 
Sign of double 
refraction. 
Crystallo- 
grapbical 
axis. 
Direction in optical ellipsoid. 
Linear coefficients of 
expansion— 
At 0°. 
At 50°. 
At 100°. 
r 
a 
Intermediate axis .... 
10-s3616 
3760 
3904 
K 2 SO 4 
Positive . . < 
b 
Second median line .... 
3225 
3366 
3507 
1 
c 
First median line .... 
3634 
4047 
4460 
Very feebly j 
■ a 
First median line .... 
3637 
3840 
4043 
Rb.>S 04 
h 
Second median line below 50° 
3214 
3398 
3582 ’ 
positive 1 
c 
Intermediate axis below 50° . 
3463 
3843 
4223 
r 
a 
Second median line .... 
3385 
3599 
3813 i 
CS 2 SO 4 
Negative . < 
h 
Intermediate axis .... 
3195 
3377 
3559 1 
1 
c 
First median line .... 
3590 
4004 
4418 
It will be apparent from the table that the relations of the linear expansions at 0° 
still hold good, as the temperature is raised, in the cases of the potassium and cmsium 
salts. A similar observation has been shown to be valid with respect to the optical 
properties. But in the case of the rubidium salt a remarkable change occurs. Owing 
to the greater increment of the expansion along the direction of the axis c, the pre¬ 
ponderance of the expansion along the axis a diminishes, until at 50° the amounts of 
expansion along these two axial directions become equal. That is to say, in the 
neighbourhood of 50° the crystals of rubidium sulphate simulate uniaxial symmetry 
as regards their thermal liehaviour. This is rendered the more interesting by the fact 
that about this temperature the crystals of this salt also exhibit uniaxial optical 
properties. Owing to the extremely feeble double refraction, it was shown [loc. cit., 
p. 693) that the slight alterations of the relations of the refractive power .in the three 
