286 ON THE EFFECTS OF PRESSURE, &C. 



The tints discovered by M. Biot along the axis of Rock 

 Crystal, and supposed to arise from a rotatory motion of the 

 particles of light, suffer no other change from pressure, than 

 that which they experience by being combined with the tints 

 produced by any other crystal. 



When the state of aggregation of the particles of doubly-re- 

 fracting crystals, is altered by the agency of heat, their polar- 

 ising forces suffer analogous changes ; but these changes are less 

 perfectly developed than in glass, owing to the great conduct- 

 ing power of regularly crystallised bodies. 



XVI. 



