74 J. P. Cooke on Tartrates of Cesia and Rubidia. 
sitive sphenoid. Crystals were also observed intermediate 
etween figs. 2 and 1 with the planes of the negative sphenoi 
in different degrees of development. The crystals of the variety 
represented by fig. 2 contained a small amount of rubidium; 
but this isomorphous admixture did not perceptibly alter the 
angles. We measured on three different crysté 
Observed. Calculated. 
+1 on +1 over vertex, r  B1° 80? 81° 30’ 
+1 on +1 over %, 51° 10/ 51° 10! 
41 on. b 139° 15/ 139° 15! 
These crystals were very perfect and comparatively large, 
measuring about 7 millimeters long by 5 millimeters wide in the 
direction of the brachydiagonal. As with the first variety, no 
accurate measurements could be obtained of the angles between 
the prismatic planes. 
2. 4. 3. 
The second type of crystals is represented by the figures 3 
and4. On these forms we have the planes of a left-handed 
sphenoid, —38, which are not found on crystals of the first type 
and are here so largely developed as to give a very different 
character to the crystal. Planes of the corresponding positive 
sphenoid were not discovered, although a large number of crys 
tals were examined. These planes were very dull and rough, 
by attaching. to them small plates of mica, and the angles 
were thus approximatively measured, but the results cannot be 
relied upon within two or three degrees. The values obtained 
were 
Measured, Calculated for 43. 
x 146° 144° 46’ 
Y,  9t4e 101° 52/ 
he Pt gee 88° 42! 
