On the di-phasic nature of glass, as shown by photo-elastic observations

In the course of an investigation on the effect of temperature upon the photo-elastic constants of glass, an specimen of flint glass was heated slowly to 400º C., and then subjected to a longitudinal pressure of 130·6 bars in a straining apparatus working on the lever principle, and very similar to that described by one of us. Normally, the load had been removed immediately after readings had been taken, but on one particular occasion the full load was left on for 42 hours, and then the specimen was allowed to cool slowly under this pressure to atmospheric temperature, the cooling process taking 7 hours. The load was then remove, and the specimen examined in sodium light between crossed Nicols with the aid of a Babinet compensator according to the method described by Filon and Jessop. It was then found that the black band in the Babinet compensator showed considerable displacement from its zero position. This was only to be expected, since naturally internal stress should be set up and the glass disannealed under these conditions.