P. Eskola — Silicates of Strontium and Barium. 347 



Fig. 8. 

 a. 1). c. 



oc 



jI 



7J/ 



Fig. 8. — Optical orientation in crystals of barium disilicate : a = crystals 

 in optical glass after N. L. Bowen; ~b and c = crystals from BaCL-flux, and 

 as formed in barium silicate melt. 



Thus these crystals are, in their habit, considerably 

 different from those in the barium crown glass, as 

 described by Bowen. In other ways my crystals agree 

 well with the characteristics given by Bowen. 



The indices of refraction are as follows : 



o(D) =1.597; £(D) =.1.612; y(D) =1.621 



As appears from these values, also confirmed by direct 

 observation, the optical character is negative and the 

 axial angle approximately 75°. A calculation from the 

 indices of refraction gives 2V = 74°45'. 



The density, determined with the pycnometer, was 

 found to be 3.73. 



The melting point of pure Ba0.2Si0 2 , according to my 

 measurements, is 1420 ± 4°. 



There was no indication that the barium disilicate takes 

 silica in solid solution; the crystals obtained from the 

 mixture containing 50 per cent Si0 2 showed exactly the 

 same optical properties as those from the pure melt. On 

 the other hand it forms a remarkable solid solution series 

 with another less acid silicate. 



Am. Jour. Sci.— Fifth Series, Vol. IV, Xo. 23.— October, 1922. 

 23 



