1098 



§ 5. The following values for tiie atomic weights of lliese elements are 

 adopted in this paper: Yttrmni -. 88,6; Lanihaniiim : 13d, ; Cerium: 

 140,23; Praseodymium: 140,6; Xeodf/miiun : 144,3; Samarium: 

 150,4; Europium: 152,0; Gadolinium: 157,3; Dijsprosium : 162,5; 

 Erbium : 167,7 ; Thulium : 168,5 ; Neoi/tterbium : 172,0; Beryllium : 

 9,1 ; Indium : 114,8; Scandium : 44,1. 



Of each salt as large a number of crystals as possible was inves- 

 tigated ; as many crystals possess more than seventy faces, the total 

 number of measurements is a very considerable one. 



Highly remarkable is in first place the extraordinary variability of 

 external aspect of the crystals of these yet strictly isomorphous com- 

 pounds. Every substituting element seems to give some preference 

 to a special aspect in most cases, and under apparently the same 

 circumstances, although eventually all the observed forms can be 

 present in all cases ; in the paper to be published at a later period 

 this fact will be discussed in detail. 



The crystals are hexagonal; a combination of the predominant 

 forms and a stereographical projection of them, are reproduced in 

 fig. 1 and 2. 



1010 



no 



1110 



Fl2. 1. 



1210 



Stereographical Projection of the observed 

 crystal forms. 



The exact determination of the class of symmetry was only possible 

 by combination of angular measurements and of RöNTGEN-photography^). 



^) I was helped in the kindest manner in this work by the valuable assistance 

 of Prof. HactA and Lecturer Dr. Ornstein, both of this university. 



