have been making expansion studies of a boric oxide glass and a sample of crystalline boric oxide using the Fizeau-Pulfrich interference method. The characteristics of the expansion curves for these two materials were found to be very different; the boric oxide glass has a "critical temperature" near 235 Β° C. followed by a very long "neck" with a softening temperature near 32o Β° C., while the crystalline boric oxide has a melting point at 45 oΒ° to 45 IΒ° C. This melting temperature for crystalline boric oxide agrees very closely with the value of 45o Β± 2 Β° C. reported by F. C. Kracek, G. W. Morey and H. E. Merwin, but differs materially from the value of 294 Β° C. offered by S. S. Cole and N. W. Taylor. The discrepancy between these two values reported for crystalline boric oxide is interpreted in light of the endothermic reaction accompanying the rapid expansion region of fused boric oxide.
The crystalline boric oxide, upon being heated to its melting point, does not return to the crystalline state when cooled, but hardens to a glass. This fact is indicated by comparison of the expansion curves obtained on specimens of the crystalline boric oxide before and after being heated to 45 IΒ° C.
The detailed report of these measurements will be published as RPI425 in the October number of the Journal of Research.