20 Wells and Penfield — Series of Caesium, Trihalides. 



jfis very common but is usually too small to give a character- 

 istic habit, and is therefore omitted from most of the figures. 

 The face g was observed only on Csl 3 , The pinacoids are vari- 

 able in their development, but commonly one and frequently 

 all three can be found on a single crystal. Pyramidal faces are 

 practically wanting. In the examination of a great many crys- 

 tals but one was found (of CsBr„I) on which a single pyramidal 

 face occurred ; this replaced the edge between m and d and 

 had the symbol 132, f — 3. The cleavage is perfect parallel to c~ 

 An idea of the similarity of the different salts may be 

 obtained from the following table, in which the axial ratios* 

 and three of the prominent angles are given. The angles 

 which were chosen as fundamental are marked by an asterisk, 

 the others are calculated, and in all cases the measurements 

 showed close agreement. 



a: b : c 



Series 

 with iodine 



Series 

 without iodine 



fCsI 3 

 | CsBrI a 

 i CsBrJ 

 | CsClBrI 



LCsCi 2 i 



CsBr s 



CsClBr, 

 CsCLBr 



0-6824 

 0-6916 

 0-7203 

 0-7230 



0-7373 



0-6873 



0-699 



0-7186 



1-1051 

 1-1419 

 1-1667 

 1-1760 

 1-1920 



1-0581 

 1-1237 



n 



a : b : 

 1-4655 

 1-4460 

 1-3882 

 1-3831 

 1-3563 



1-4550: 



1-430 



1-3917 



1-6196 

 1-6511 

 1-6196 

 1-6268 

 1-6167 



1-5395 

 1-5638 



Csl s 



CsBrI„ 



CsBr.I 



CsClBrI 



CsClJ 



CsBr, 



CsClBr 



GsCLB'r 



m^m, 1 10 ^ 110 

 *68° 37' 

 *69 20 

 *71 32 

 *71 44 

 72 48 

 *69 

 *69 56 

 *71 24 



d^d, 011 A Oil. 

 *95° 43' 

 97 34 

 *98 48 

 *99 15 

 *100 1 

 *93 14 



*96 40 



Cab, 102 a 102. 



78° 0' 



*79" 5 



78 



78 15 



*77 54 



75 10 



76 



Csl 3 and CsBr 3 are almost identical in axial ratios, but it is 

 surprising that the chemically intermediate compounds, CsBrI 2 

 and CsBr a I, are not crystallographically intermediate. 



A very remarkable relation exists between the first five 

 compounds in the table, all of which contain iodine and are 

 arranged in the order of their molecular weights. The ratio 

 of two axes remains nearly constant throughout, while the 

 third varies.f Exactly the same relation exists between the 



* The ratios are given in two ways: I, with b as unity, as is customary; and II r 

 with & as unity, in order to show more clearly the relation between & and c. 



f Several series of organic compounds with analogous axial relations have been 

 observed by Groth (Berichte, iii, 449) and by Hintze (Berichte, vi, 593.) 



