16 Blake — Some Isomorphous Triple Thiocyanates. 







Develop- 













Molec. 



ment of 









Bire- 



Salt. 



weight. 



P (111). 



c 



0) 



e 



fringence. 



Cs-AiT-Ba 



1042-5 



Absent 



0-9003 



1-7761 



1-6788 



0-0973 



Cs-Cu-Ba 



998-2 



Slight 



0-9183 



1-8013 



1-6882 



0-1131 



Cs-Ag-Sr 



993-3 



Slight 



0-9165* 







--- 



Cs-Cii-Sr 



949-0 



Large 



0-9158 



1-8535 



1-6982 



0-1553 



The salts have been arranged in order of decreasing molecu- 

 lar weight. It will be noticed that as the heavier metals are 

 replaced by lighter ones, both indices of refraction, as well as 

 the birefringence, grow successively greater, although the oppo- 

 site behavior might have been expected. The development 

 of the sphenoidal form, ^(111), shows a similar regularity; 

 whereas the lengths of the vertical axis show the inverse rela- 

 tion, except for that of the silver-barium salt. Hence one 

 might conclude that : (1) Similarity of the atomic weights of 

 the basic elements, as in the case of the csesium-silver-barium 

 compound, tend to decrease both the indices of refraction and 

 the birefringence, and also to suppress the sphenoidal form ; 

 (2) ]^either the chemical composition nor the optical proper- 

 ties bear a simple relation to the length of the vertical axis. 



Attempts to obtain other members of the series for further 

 comparison, as by substituting rubidium for caesium, have so 

 far been unsuccessful. 



The kind advice and supervision of Prof. S. L. Penfield is 

 gratefully acknowledged ; also the kindness of Prof. H. L. 

 Wells and his assistants in supplying the crystals examined. 



Mineralogical Laboratory of Sheffield Scientific School, 

 Yale University. 



* Eepeated attempts to obtain the indices of refraction were unsuccessful, 

 apparently on account of the great solubility of the material and the conse- 

 quent difficulty of separating the crystals from the concentrated mother 

 liquor in good condition for crystallographic study. The crystal faces were 

 always somewhat uneven, and the reflections consequently blurred ; hence 

 the length of the vertical axis, also, is less reliable than the values given for 

 the other salts. 



