185 



€i.re composed of prisms varying in thickness, and, of course, in tint, 

 and have often rims, formed of one or two concentric bands, made up 

 of radiant bunches, proceeding from the inner margin of the bands, 

 -and not from the centre of the discs. The large discs exhibit ten, 

 twelve, or more fine concentric lines, which are lines of cleavage. 

 Sometimes the rim is as wide as the inclosed part, and these polarize 

 a bluish white, 



3. Asparagine. — This substance yields discs resembling those of 

 salicine, but are still more brilliant and beautiful. There are some 

 discs which exhibit no circular polarization, and others which exhibit 

 a succession of black and white narrow rings, like those seen round 

 the star Capella, with annular apertures. 



4. Man7ia. — This gives fine negative crystals, both by fusion and 

 solution. There is great brilliancy and uniformity of the tints, and 

 the black cross is so sharp that its intersection is not easily seen. 

 The discs form a united hexagonal mosaic, and have no rims. 



5. Disulphate of Mercury . — The solution of this salt in nitric acid, 

 gives, by slow cooling, square crystals with circular polarization, which 

 undergo singular modifications, for which we must refer to the paper. 



6. Parmeline^ from alcohol, gives fine circular crystals. 



7. Palmic Acid, by fusion, gives fine negative circular crystals, 

 like the mosaic of manna. 



8. Nitrate of Uranium gives fine negative circular crystals, from 

 water, alcohol, ether, and oil. 



9. Palmine gives very minute circular crystals. 



10. Chromic Acid gives very peculiar circular crystals, composed 

 of concentric rippled bands, generally of the blue of the first order. 



1 1 . jBer^erme gives negative circular discs, resembling those of 

 oil of mace. 



12. Sulphur et of Cadmium, dissolved in nitric acid, that is, nitrate 

 of cadmium, gives beautiful negative circular crystals. 



13. Sulphate of Ammonia and Magnesia. — This salt yields fine 

 positive circular crystals. 



14. Hatchetine, Cacao Butter, White Wax, Tallow, Adipocire, 

 and all Soaps, and difi^erent kinds of Fat, give circular crystals like 

 oil of mace. 



15. Borax in Phosphoric Acid. — This salt yielded the circular 

 crystals described by Mr Talbot. This salt, as well as nitrate of 

 uranium, yields hemispherical bells, under certain circumstances, which 



