72 Mr. A. E. Tuttn. A Comparative OryftaUographical 



of temperature. Measurements in succession at 10'5 and 60'5 indi- 

 cated that 2E increases 30' for 50 of rise of temperature. 



C/ESIUM ZINC SELENATE, 



The analysis of a specimen of the crystals of this salt afforded the 

 following numbers : 0*7027 gramme gave 0'0790 gramme of ZnO, which 

 corresponds to 9 -02 per cent, of zinc. The calculated percentage is 

 8-98. 



Goniometry. 



Eleven of the most suitable small crystals, derived from five dif- 

 ferent crops, were employed in the goniometrical measurements. 

 Habit : flattened prismatic. 

 Axial angle : (3 = 73 49'. 

 Katio of axes : a : b : c = 0'7314 : 1 : 0'4971. 



Forms observed: b = {010}oo^oo; c = {001 }oP; p = {110}ooP; 

 q = {011}?oo; 7?i = {021}2?oo; o' = {Ill}+P; 

 r' = 



The results of the measurements are presented in the accompanying 

 table of angles. 



The habits of the crystals of the various crops are typified by the 

 following figures of the double sulphate memoir (loc. cit.) : figs. 9 and 

 10 in the description of caesium zinc sulphate, fig. 16 in the descrip- 

 tion of caesium ferrous sulphate, fig. 21, but with much larger q faces, 

 and fig. 34 in the description of caesium cadmium sulphate as far as 

 regards the relation of the c, r and q faces. They are characterised 

 by large q faces relatively to the faces of the basal plane, which latter 

 is usually only represented by a mere strip. The faces of the hemi- 

 pyramid o' are often considerably developed. 



The clinopinacoid b is frequently present, but the orthopinacoid a 

 was never observed. The clinodome m was found developed on one of 

 the crystals measured. 



The cleavage parallel to /{201}, common to the series, was well 

 developed. 



