74 J. P. Cooke on Tartrates of Cesia and Rubidia. 
the crystals of the bitartrate of cesia. As is shown by the fig- 
ure, the planes 1? are more largely developed on the crystals of — 
the rubidium than those of the cesium salt, and in this as wellas 
in the other figures, we have endeavored to preserve as nearly 
as possible the general habitus of the crystals, as well as the 
relative dimensions of their planes. . a 
The cleavage of the crystals of the bitartrate of rubidia 18 
in all respects similar to that of the cesium salt, and the same 1s 
true of the crystals formed by an isomorphous mixture of the 
two substances. Moreover, the playes 7z and #7 are similarly 
striated on both. : 
3. Bitartrate of Potassa.—We add for the sake of comparison 
the elements of the crystalline form of the ordinary bitartrate of 
potassa as determined by Schabus (‘ Rammelsberg’s Krystallo- 
graphische Chemie,” page 304). His results, reduced to the system 
of notation used in this article, are 
“ a:b :c=0°7372 :1:0°7115, 
X= 100° 20, Y¥ == 125° 46/, Z= 108° 38", 
For the most pS the same planes occur as on the crystals ; 
