164 ON THE OPTICAL PROPERTIES OF MURIATE OP SODA, &c. 



sage, shew still more satisfactorily the difference between the 

 two classes of phenomena. 



I do not understand what M. Biot means by saying, that 

 these discoveries were made by accident, (" observees accidentel- 

 " lenient 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 polarisino- 

 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. Malus and M. Biot 

 I had sought for it in vain in ordinary specimens. 



IX, 



