ON POLARIZED LIGHT. 195 



&c. 7 to preserve them from injury by the pressure of the glass 

 cover), or in Canada balsam. If it be desired to examine the 

 crystals during their formation, the crystallization should be 

 carried on in a glass that is slightly concave. All those crys- 

 tals that are so thin as not to exhibit color, may have color 

 given them by placing a film of mica or selenite under them 

 on the stage of the microscope. 



According to Mr. Fox Talbot, who first applied the micro- 

 scope to the examination of polarized light, sulphate of copper, 

 crystallized from a solution to which a little nitric ether has 

 been added ; oxalate of chromium and potash, from an aqueous 

 solution ; and borax, crystallized in dilute phosphoric acid, are 

 especially beautiful. 



