;! 1 S Rankin and Meriom — 



solid solution. 7 The birefringence, y-a as measured 

 independently on several prisms is -003--004. 



The optical method of determining the composition of 

 an unstable compound consists in obtaining from a glass 

 of proper composition at a temperature below that of 

 dissociation, an optically homogeneous crystalline aggre- 

 gate having optical properties corresponding to the 

 optical properties of crystals which are stable in the field 

 of the compound. The desired aggregate should be 

 obtainable, provided minute porosity does not develop 

 during crystallization. In the case in hand we assume 

 that there is no porosity, for there appears to be 

 expansion instead of contraction during crystalliza- 

 tion (the crystals have lower refraction than the glass). 

 Glasses having the compositions MgO 11-13, A1 2 3 

 31-29, Si0 2 57 crystallize to aggregates with a 

 mean refractive index of 1-521 ±-003, that is, like 

 crystals formed in the most siliceous melts within the 

 field of the ternary compound. Likewise glasses of com- 

 positions MgO 13-15, A1 2 3 35-33, Si0 2 52 give n = 

 l-527±-003, like crystals from the least siliceous melts. 

 The only simple ternary compounds approximating these 

 compositions are MgO.Al 2 3 .3SiO, (MgO 124, A1,0 3 

 31-6, SiO, 56) and 2Mg0.2Al 2 3 .5Si0 2 (MgO 13-7, 

 A1,0 3 34-6, Si0 2 51-7). 



What were apparently very nearly homogeneous 

 crystalline aggregates were prepared from glasses of a 

 considerable range of composition extending on the com- 

 position diagram as a thin lens-shaped area, with its long 

 axis the silica-spinel line, and one curved boundary 

 nearly coinciding with the boundary curves 7-3-2. This 

 area covers a range of silica content from about 45 to 70 

 per cent. In fact, if homogeneity of such aggregates had 

 been the sole method of determining the composition of 

 this unstable crystalline phase, the conclusion that it was 

 a solid solution over a long range of compositions would 

 have been reached. 



There are three chief points to be considered in the 

 interpretation of such aggregates: (1) the absolute sizes 

 of grain of the crystals. (2) the shapes of the crystals, 

 (3) the relative refractions of the crystals and possible 

 glass. Obviously, whatever the other conditions, if the 

 constituents have nearly the same refractive index, or if 



7 Compare the case of spinel, J. Am. Chem. Soe., 38, 568, 1916. 



