ON THE OPTICAL PHENOMENA OF CRYSTALS. 221 



All the circles, when viewed by common light, appeal- 

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

 posed of acicular 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 



