286 ON THE EFFECTS OF PRESSURE, &c. 
The tints discovered by M. Bior 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. 
