Blake Some Isomorphous Triple Thiocyanates. 15 



Cs^Ag,Sr{SCN)^, 



The habit of this salt is the same as that of the silver-barium 

 compound, except that the sphenoidal faces, ^(111), were 

 observed on two crystals out of twenty, as in figure 2. The 

 reflections were only fair at the best, although fifty or sixty 

 individual crystals, obtained from four or five different crops, 

 were examined. Hence the optical data obtained are value- 

 less for comparison with those of the other salts studied, 

 although they are sujfficiently characteristic to bring this salt 

 clearly within the series. The following angles were measured : 





Measured. 



Calculated 



q^q\ 201^201 



q^q\ 201/^021 



m^q, 110^201 



c^2^, 001^111 



122° 46'* 

 76° 35' 

 51° 42' 

 52° 29' 



76° 44t^' 

 51° 38' 

 52° 21' 



Vertical axis c — 0*9165. 



C\Ca,Sr{SCN'),. 



The habit of these crystals is much like that of the copper- 

 barium salt, but the sphenoidal faces, ^(111), and the base, 

 c (001), are generally more largely developed than in figure 2, 

 the p faces being often as large as the faces of the pyramid of 

 the second order, q (201). Indeed, the sphenoidal faces are 

 more pronounced and more generally present on this than on 

 any other salt of the series. The following angles were meas- 

 ured : 



Measured. Calculated. 



122° 44'* 



76° 41' 76° 43^' 

 51° 39' 51° 38' 

 52° 18' 52° 20' 



Vertical axis g = 0*9158. 



Indices of refraction for sodium light : 



0)= 1*8535; e= 1*6982. Difference = 0*1553. 



Comparative Study. 



It will be observed that the indices of refraction are high, 

 and that the birefringence is likewise high. Further, the rela- 

 tive values of the indices of refraction for the ordinary and 

 extraordinary rays confirm the evidence obtained by use of the 

 mica plate that the material is optically negative. The regu- 

 larity of the physical and chemical variations is made evident 

 in the following table of collected results : 



^^a\ 



201/^201 



<l^<l\ 



201/^021 



m^^q. 



110/s201 



c^p, 



001/slll 



