618 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
Other three rings follow in succession, the shite tint rising to the yellow, and again 
falling to its original colour. Each of these five rings have precisely the same 
tints through .ut their circumference, and when a number of such crystals appear 
in the dark field, they form objects of singular beauty. 
In some specimens, the discs have the appearance of cones, as in Mannite. 
They have, in the centre of the black cross, another cross whose arms bisect the 
sectors, having sometimes a white, or yellow, or green tint. This cross is surrounded 
with a faint ring, which separates it from large sectors of a bright pink colour. 
The circular discs are often composed of radial lines of different thicknesses, 
and in imperfect optical contact. Their tints consequently vary throughout the 
disc, and have a remarkable appearance. When the crystals are very small, they 
produce the polarised halos given by oil of mace. 
19. Hippuric Acid.—This salt gives imperfect discs when melted. With water, 
it gives good circular crystals, but very fine ones with alcohol. They have a great 
variety of forms and tints, depending on the strength of the solution; but they 
differ from other circular crystals in two points. The radial lines are often sepa- 
rated from one another by black spaces of the same breadth as the luminous 
radial lines, and the whole disc is covered with almost invisible concentric black 
circles, at equal distances from one another. They are seen most distinctly in the 
white field. The four central sectors are often surrounded with a ring separated 
from them by a black space entirely free from matter. In some specimens, the 
discs consist of eight or ten sectors of uniform thickness and tint, which become 
black when in the plane of primitive polarisation. In other specimens, the crys- 
tallisations are large, irregular, and highly coloured. 
Having thus described the phenomena exhibited by some of the more impor- 
tant circular crystals, I shall give a tabular list of the other substances in which I 
have found the property of giving circular crystallisations, arranging them under 
the heads of Positive and Negative, as formerly explained. 
Positive Circular Crystals. 
Sulphate of ammonia and magnesia. Sulphate of iron and ammonia, 
red oxide of manganese. as wa potash. 
Hydrate of potash. stile manganese and ammonia. 
Citrate of potash. rit magnesia and ammonia. 
Muriate of morphia. ae zine and potash. 
magnesia, Disulphate of mercury. 
Almond soap. Mannite. ' 
Starch, Citrate of ammonia. 
Substance in garnet. Myristic acid. 
30 mica Cuprose sulphate of potash. 
Chloride of strontian, Kreatinine. 

Sulphate of cobalt and ammonia. 
Se 
