DOUBLE REFRACTION IN ISOMORPHOUS SUBSTANCES. 293 



micas. Physical investigation is with regard to them in advance 

 of chemical. However, there is no doubt that careful examina- 

 tion would lead to the recognition of differences in composition, 

 which are indeed indicated by the differences of colour. It 

 would be very interesting to find modes of investigation so dis- 

 similar mutually confirming each other ; and in the same point 

 of view, it would be of great importance to seek in the plumbi- 

 ferous arragonitcs for the convergence, and perhaps the disper- 

 sion of the optical axes, which the above experiments would 

 appear to point out and lead us to expect. 



