236 INTERFERENCE OF POLARIZED LIGHT. 



Mitscherlich found that, in sulphate of lime, the angle be- 

 tween the axes (which is about 60 at the ordinary tempe- 

 rature) diminishes on the application of heat; that, as the 

 temperature increases, these axes approach until they unite ; 

 and that, on a still farther augmentation of heat, they again 

 separate, and open out in a perpendicular plane. Heat is found 

 to dilate this crystal more in one direction than in another 

 perpendicular to it. 



