616 SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 
The solutions of this substance in muriatic and acetic acids gave no circular 
crystals. 
14. Sulphate of Ammonia and Maygnesia.—When an aqueous solution of this 
salt is brought into a viscous state by heat, and slowly cooled, very beautiful cir- 
cular crystals are formed, sometimes large, and sometimes smaller than the 
1000th of an inch. Round some of them there is formed a radiant crystalline 
halo, separated by a black circle from the disc which gives the four luminous 
sectors. The polarisation of the crystals is positive, and they are perfect at the 
centre, unless when large and badly formed. 
15. Hatchetine, Cacao Butter, White Wax, Tallow, Adipocire, and all Soaps 
and different kinds of Fat, produce circular crystals like O7l of Mace, and give the 
same halos, which, in the polariscope, are divided into four sectors. 
16. Borax in Phosphoric Acid.—The crystals produced by this combination 
are those discovered by Mr Tarot, and have been already described. I mention 
them again, in order to notice the interesting hemispherical bells which I have 
observed when an aqueous solution is raised into froth by heat. These bells or 
bubbles indurate and polarise the light by refraction, as in the case of nitrate of 
uranium already mentioned. They are traversed by the black cross, and exhibit 
rings of colour, which are green at the centre, then red, then green and red again. 
Some of these bells contained smaller ones within them, in one case no fewer 
than eight; and in one of them I distinctly observed a crystalline structure, the 
minute crystals radiating from the apex of the bell. 
17. Mannite—This substance gives circular crystals with more facility and 
certainty than any other which I have examined. When Mannite is melted by 
heat, it gives beautiful circular crystals. When dissolved in water they are very 
good. They are not good in alcohol or ether ; but in acetic acid the finest circular 
crystals are formed. The black concentric circles are peculiarly fine, and are, so 
far as the microscope can shew it, entirely free of matter. In the crystals from 
acetic acid, the sectors shade off into the arms of the black cross with such per- 
fection, that the circular disc loses its flat appearance, and seems to be composed 
of four solid cones, whose apices meet in the centre. In place of being circular 
the crystals are sometimes drawn out, as it were, into long cones, as shewn in 
Fig. 19, rounded at their summits, and having the appearance of solids of that shape. 
The black cross appears at the summit of the rounded cones, one of its arms, and 
sometimes two, according to the position of the plane of primitive polarisation, 
stretching out to the termination of the rounded cone. These cones are often 
crossed by two, three, or four concentric arches, perfectly black. In these elon- 
