194 DR BREWSTER on a New Species of Double Refraction. 



the crystal is a combination of solids of variable density, and se- 

 parated from one another by numerous intersecting planes or 

 nodes, where the variations of density change their direction, 

 we may ascribe to this cause the difficulty with which friction 

 decomposes the natural quantity of electricity which resides 

 in the mineral. 



