70 J. P. Cooke on Tartrates of Cesia and Rubidia. 
Hague, the ratio is nearly 1:2:3, giving the formula Mg%St 
+2Mn°Si or (}Mg+23In)°Si. The specimens analyzed by Deville 3 
and myself, as well as those investigated by Thomson and Ram: — 
melsberg, are very nearly pure Mn®si, so that we have here repre: 
sented three distinct varieties of tephroite, each giving a simple 
ratio and formula. The replacement of manganese by magnesia, 
as shown by the above results, is exceedingly interesting, in view 
of the fact, that both chrysolite and tephroite crystallize in the 
trimetric form. A further analogy is observed, when the varie 
ties of tephroite are compared with those of chrysolite; for 
besides the indefinite isomorphous mixtures of magnesia and 
iron in the various olivines, we have in bollonite an example of 
a magnesian chrysolite, and in hyalosiderite an iron magnesia 
chrysolite, (Fe®Si+Mg°Si), while fayalite is an almost pure Irony 
chrysolite. The analyses of tephroite, here given, seem to ‘ 
monstrate that the varieties thus far examined have no oxy‘ 
of zinc in chemical combination, although the mineral is intr 
mately associated with both zincite and willemite. : 
New Haven, Oct. 1st, 1863. 
Arr. VUI.—Crystallographic Examination of the Acid Tartrates of 
Cesia and Rubidia; by Jostan P, Cooxe, Jr. 
1. Bitartrate of Cesia, HO, CsO,C,H,0, ,.—This salt forms 
transparent and colorless crystals belonging to the oe 
system, which may present either a right-handed or a left-hand 
hemihedrism. ‘The axial relations calculated from the aD 
Zand Y of the fundamental octahedron are ; 
a:6:¢=>0661:1 : 0-694 
The observed planes were 
+1 +4 (a: 5:6) @i—aa: 6: ac : 
~1=~$ (6:8: ¢) to wes abs ec 
4 * 
~$3= — 4($a: 5: 3c) Me ae Ps ce 
to aac bie 
The values obtained for the angles are as follows. Those 
asterisked were used in calculating the angles given in the sec 
ond col e aumann and Dana, X indicates the 
angle between two planes of the fundamental octahedron over 
the macrodiagonal edge, Y the angle over the brachydiagonal 
edge, and Z the angle over the basal edge. 
X= 109° Y = 128° 50’ Z—= 98° 30’ 
