276 Dr BREWSTER on a remarkabk peculiarity in Glauberite. 



tint between the two resultant axes diminished with the heat of 

 the hand. I immediately increased the temperature, and before 

 it reached that of boiling water, the weaker axis for red light 

 disappeared altogether, so that the crystal had only one axis for 

 red light. The axis, however, re-appeared, but the plane pass- 

 ing through the resultant axes was now at right angles to what 

 it was at first. 



By the application of artificial cold, a new axis was created 

 for violet light, and the plane of the two resultant axes coincided 

 with the plane of the two resultant axes for red light at the or- 

 dinary temperature. 



Results analogous to these have been obtained by Professor 

 MITSCHERLICH for other minerals ; but I am not aware that he 

 has observed such marked changes produced by such a slight in- 

 crease of temperature, or that he has made any observation at 

 all upon Glauberite. As the subject belongs to him, I have 

 merely noticed the very singular fact which so unexpectedly pre- 

 sented itself. 



