164 ON THE OPTICAL PROPERTIES OF MURIATE OF SODA, &c. 
sage, shew still more satisfactorily the difference between the 
two classes of phenomena. . 
I do not understand what M. Bror means by saying, that 
these discoveries were made by accident, (“ observées accidentel- 
“ Jement dans certains echantillons’’); for all the results in the pre- 
ceding paper were obtained in the course of a regular train of ex- 
periments, undertaken for the express purpose of examining 
this anomalous class of crystallised bodies; and the polarising 
structure was detected, not in some specimens, but in every 
specimen of a proper thickness which I had an opportunity of 
trying. 
In my experiments on Animal Jellies, I had repeated oppor- 
tunities of observing, that a polarising structure, invisible in a 
single mass, could be rendered perceptible by an augmentation 
of thickness ; and I was naturally led to suspect, that a similar: 
result would be obtained by using large masses of muriate of 
soda, and fluor-spar, although, like M. Matus and M. Biot, 
T had sought for it in vain in ordinary specimens.. rita 
TX. 
