ON THE OPTICAL PHENOMENA OF CRYSTALS. 221 



All the circles, when viewed by common light, appear 

 transparent, white, and very uniform. If they are com- 

 posed of acicnlar crystals diverging from a point, these 

 latter must be exceedingly slender and numerous, and in 

 perfect optical contact, since a high magnifying power 

 does not render them separately visible. 



Sect. 2. — With respect to the chemical nature of 

 these crystals, it appears to me evident that they consist 

 of boracic acid. They are obtained by dissolving borax 

 in phosphoric acid ; and it may be inferred that this 

 latter substance unites with the alkali, and isolates the 

 boracic acid. In order to see if this supposition were 

 correct, I dissolved boracic acid in alcohol, and I found 

 that a drop of this solution evaporated on a plate of glass 

 frequently yielded an abundant crop of the crystalline 

 spherules. But these are generally exceedingly small, 

 requiring a high power to display in them the cross and 

 four sectors of light, and they speedily grow opaque ; for 

 which reason they are not so well suited for observation 

 as those prepared by the former method. They establish 

 the fact, however, that this mode of crystallizing is a 

 property of the boracic acid. It is highly improbable 

 that it should be peculiar to that substance, but I have 

 not yet met with it in any other. 



Sect. 3. Explanation of some of the Optical appearances. 

 — 1. When any doubly refracting crystal is examined 

 with the polarizing microscope, (the polarizers being 



