Study of the Double Selenates of the Series E.,M(Se0 4 ) 2 ,6H 2 0. 79 



The minutes figures in brackets in the table are the observed values 

 in those cases where the differences between the observed and calcu- 

 lated values are appreciable. 



The angular change is not generally directly proportional to the change 

 in the atomic weight of the alkali metal, the maximum variation from 

 direct proportion among the primary angles being exhibited by the 

 prism zone, where the changes in ap or bp are as 1 to 2. 



Comparison of the Axial Ratios 



For potassium zinc selenate, a : b : c = 0*7458 : 1 : 0*5073 

 rubidium a : b : c = 0*7431 : 1 : 0-5019 

 caesium a : b : c = 0-7314 : 1 : 0-4971. 



Comparison of the Relative Densities. 



Potassium zinc selenate 2*5537 



Diff. 0-3067 



Rubidium 2-8604 



Diff. 0-2549 



Caesium 3-1153 



The difference between the densities of potassium zinc and rubidium 

 zinc sulphates is 0*343, and between those of the latter salt and caesium 

 zinc sulphate 0-283. 



Comparison of the Molecular Volumes. 



Potassium zinc selenate 210*13 



Diff. 9-77 



Rubidium 219-90 



Diff. 12-50 



Caesium 232-40 



The differences for the two replacements in the zinc double sulphates 

 were 9*52 and 12*68. 



Comparison of the Distance Ratios. 



Diff. 



Diff. 



v. Diff. 



KZn selenate . . , 

 KhZn 

 CsZn 



6-1941 

 6 *3062 

 6 -3860 



1121 



798 



1919 



8 -3054 

 8 -4363 

 8*7311 



1809 

 2448 

 4257 



4-2133 

 4 -2593 

 4*3402 



460 

 809 



1269 



