OPTICAL PHENOMKNA OF CRYSTALS. 227 



1 will now mention some other crystals which possess 

 the analyzing property, but not in such a high degree. 



2. Boracic acid. — If dissolved in boiling water, it yields 

 in cooling irregular crystals which have considerable 

 analytic power. A crystal which in one position is so 

 translucent as to be hardly distinguishable from the 

 water in which it floats, is in the transverse position 

 very strongly defined. It does not become dark all over, 

 but only in its outline. 



If now we employ it to analyze the tints of sulphate of 

 lime, its outline becomes beautifully coloured. Nothing 

 can exceed the delicacy of colouring which a number of 

 these crystals exhibit when viewed together ; those which 

 lie in one direction appearing, for instance, green; those 

 in a transverse direction, red. The appearance is very 

 unlike any other optical phenomenon that I know of, in 

 consequence of two colours being seen in strong contrast, 

 and without any intermediate tints ; and also from the out- 

 line only of the objects being coloured, while their interior 

 remains without colour. It is only when the crystals 

 have a fibrous or striated structure that the tint extends 

 over all their surface. 



The boracic acid has the same analytic property, and 

 precisely the same appearance, when it crystallizes from 

 a solution of borax in phosphoric acid. The plumose 

 crystals of it (No. 4, supra) are very delicately coloured 

 with the two opposite tints. 



I obtained a very beautiful result by placing a drop of 

 phosphoric acid upon a group of circular crystals. This 



