
SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 615 
composed glass. After the deliquescence of the crystals, I attempted to make 
another crop, but having failed, I set the piece of glass aside. In the course of 
half-an-hour, however, I found it covered with a fine and splendidly-coloured set 
of circular crystals, which dissolved wholly when placed in castor-oil with the 
view of preserving them. The light polarised by the bells above mentioned, 
formed a double ring, ved on one side, and gzeen on the other, with a black space 
between. 
Upon examining the solution in castor-oil, after having stood upwards of four 
years, I find that circular crystals of three different kinds have been formed, 
some small and very perfect, with four sectors and no rim; others with broad 
rims, with quaquaversus polarisation; and a third set in which the structure 
has been entirely decomposed, and the circular form of the disc preserved. 
10. Palmine.—This substance melts like tallow into a uniformly luminous 
film, apparently with quaquaversus polarisation; but upon examining it with a 
high power in the polarising microscope, it exhibits millions of circular crystals, 
each bearing its little black cross. These crystals are so minute as to produce 
splendid halos, which, in the polariscope, give four luminous sectors exactly like 
those in oil of mace. 
11. Chromic Acid.—The circular crystals of this substance, dissolved in water, 
are of a very peculiar kind. They are negative, and are very imperfectly repre- 
sented in Fig. 11, where the circular disc is composed of a great number of con- 
centric circles, whose tint is the blue of the first order, rising, in some cases, to 
the yellow of the same order. These circles may be described as rippled lines con- 
sisting of minute crystals, separated by others still more minute, and incapable of 
polarising the light. The system of concentric rings is traversed by the usual black 
cross. This salt gives another kind of crystals, in which are separate concentric 
rings without the black cross, and consequently with quaquaversus polarisation. 
12. Berberine.—This salt gives very fine circular crystals which are negative, 
and form beautiful halos like those in oil of mace. The ordinary crystals often 
form a number of crystalline rings in contact, each of which contains circular 
crystals of different sizes, and occasionally prismatic crystals along with them. 
13. Sulphate of Cadmium—tThe sulphuret of cadmium, dissolved in nitric 
acid, is converted into sulphate, which gives beautiful negative circular crystals, 
varying from the 800th of an inch to the 3000th. After the sulphuret is melted, 
and the acid driven off, no crystallisation is seen, but in an hour or two a deli- 
quescence takes place, and the circular crystals gradually appear. There are 
many of them so small and thin, that they have no action on polarised light. 
VOL. XX. PART Iy. 8D 
