280 CRYSTALTJZATION. 



is miicli increased by the crystallizutiou. Eft'ects comparable with these 

 may be produced by mechauical means, A cube of rock salt, which 

 has no efi'ect on plane-polarized light in its ordinary state, changes the 

 plane of polarization when it is compi-essed in a vise. And a cleavage 

 slice of jDrussiate of potash, which is uniaxial, may by compression be 

 distorted so as to gi^'e in a convergent beam of polarized light elliptical 

 rings and two eyes like a biaxial crystal. 



