Page:Collected Physical Papers.djvu/135

Rh I now tried to devise some experiments strictly analogous to the optical experiments with chilled glass. I obtained extremely good results with a cylinder of cast ebonite, in which the stresses present are exactly similar to those in a circular piece of unannealed glass.

The next series of experiments were undertaken with mineral specimens. One very interesting object obtained from Egypt was formed by ringed concretion of flint round a central nodule. This specimen exhibited the cross with great distinctness. I also obtained fairly satisfactory results with stalactite. The concretion of calcium carbonate formed inside a pipe by deposits from temporarily hard water flowing through it, would also be found to exhibit this phenomenon. (Proc. Roy. Soc., March 1898.)