J. P. Cooke on Tartrates of Cesia and Rubidia. 71 
4 
Observed. Calculated. 
+1 on +1 over vertex, 81° 30/* 
+1 on +1 over 7, 51° 10/* 
+1 on —1 over brachy.-edge, 128° 58! 128° 50! 
+1 on —1 over macro.-edge, 102° 58! 103° 
+lon [ 139° 15! 139° 15/ 
+lon 115° 35/ 115° 35/ 
Ton I over %, 69° 22' 69° 80/ 
lion 1% over vertex, 118° 4 
These angles were measured on three different crystals similar 
to fig. 1, and excepting for the angles between 1 
the prismatic planes, the values closely agreed gee? 
on all. The planes +1 were very perfect and 
the angles between them agreed to a min- 
ute. The planes —1 were not so perfect, but 
the angles which they formed are accurate as 
given above within a few minutes. The planes 
v1 and 72 were strongly striated parallel to the 
vertical axis and the angles made by them 
with other planes could not be measured with 
any accuracy when the intersection edge was 
parallel to the direction of the striation. The 
Same was also true of the angles made by 
the planes I, under the same circumstances, 
although no striation was visible and the re- 
When, however, the intersection-edge was at 
right angles or greatly inclined to the strize, the 
angles could be measured within a few minutes, and were found 
to be very constant. The planes 17 on all the crystals examined 
were very imperfect and generally only rudimentary, : 
he crystals of the bitartrate of czsia cleave with great readi- 
ness parallel to the plane 77, with less readiness, but still easily, 
parallel to 77, giving in each case brilliant planes of cleavage at 
right angles to each other. No evidence of cleavage parallel to 
the basal section could be detected, the erystals when broken 
or split in this direction always giving a conchoidal fracture. 
Among the crystals of this salt kindly submitted to our ex- 
amination by Mr. Allen, two very different types of forms were 
easily distinguished, which, as we are informed, were the result 
of wholly different crystallizations. In fig. 1 we have both the 
Positive and negative sphenoids (which form together the funda- 
mental octahedron), the planes of the first being distinguished 
from those of the last only by being uniformly much more de- 
veloped and having a greater brilliancy. In another variety of 
this same type of forms, represented by fig. 2, we have only the 
