bowen: barium disilicate in glass 267 



The optical properties correspond with those of no crystals 

 hitherto described. In casting about for a possible composition 

 for the crystals the orthorhombic symmetry of K2Si205 was 

 recalled together with the general tendency towards isomorphism 

 of potassium and barium compounds and it was thought possible 

 that the crystals might be BaSi205. Accordingly a mixture of 

 that composition was made up and melted in a platinum crucible 

 and this was found to give on cooling a homogeneous crystalline 

 mass having optical and crystallographic properties identical 

 with those exhibited by the crystals in the glass. The refractive 

 indices were determined on this pure material, on which they can 

 be determined with greater accuracy than on the very thin 

 crystals embedded in glass. They are as follows: 7 = 1.617, 

 a = 1.598. 



It is probable that the slightly lower values given for the 

 crystals in the glass represent a real difference and that when 

 formed from the glass they take a small amount of alkaline disili- 

 cate into solid solution, but this cannot be definitely decided 

 without measurements on larger crystals that will permit of 

 greater accuracy. The very minute crystals formed by devitri- 

 fication of the glass at low temperatures appear to have indices 

 that are even somewhat lower still, suggesting that they have a 

 larger amount of 'alkaline disilicate in solid solution. The 

 probability of solid solution between barium disilicate and potas- 

 sium disilicate is very great, for they show a striking degree 

 of isomorphism, the corresponding angles of potassium disilicate 

 being, within the limits of error of measurement under the micro- 

 scope, exactly the same as those here found for barium disilicate.^ 

 In subsequent work a detailed study of solid solution between 

 these compounds wdll be made. 



Barium disilicate appears not to have been prepared and studied 

 before and this note has been written to place its properties on 

 record. To make it more complete the melting point has been 

 determined. It was found to melt congruently at 1426°C., 

 as determined with a platinum-rhodium thermo-element and 

 potentiometer system. The only reference in the literature to 



* See crystals figured by Fenner in a paper by G. W. Morey (New crystalline 

 silicates of potassium and sodium. Journ. Amer. Chem. Soc. 33: 228, fig. 4. 1914.) 



