
SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 617 
gated crystallisations, the elementary particles are in perfect optical contact, the - 
tint which they produce being a bright white of the jirst order. At this period 
of their formation, a crystallised and semi-opaque crust is formed above many of 
them, the opacity arising from an imperfection in the optical contact. This crust 
sometimes cracks and falls off, leaving the perfect crystal beneath, or when it 
merely cracks, shewing the perfect crystal through the fissure. These incrusta- 
tions sometimes occupy the middle of the spaces between the black arches, m n, 
o p, &¢., and raise the tint to an orange-brown. In a specimen preserved in 
Canada balsam, the balsam has insinuated itself between the imperfectly-united 
elementary crystals, and made the crust so transparent, that the crystal beneath 
it is most distinctly seen, as if through a piece of glass. 
In some specimens, the optical contact is so imperfect, that groups of discs 
have a pale nut-brown semi-transparency, with the concentric black bands finely 
developed. 
In other specimens, we have every degree of transparency, up to absolute 
opacity. In some discs, the black cross is scarcely seen, and they seem as if they 
were composed of fine threads of worsted, from the sides of which other finer 
threads diverge. Such crystals are beautifully white by reflected light, and look 
as if they were formed of fibres of white satin. 
An interesting peculiarity in the larger discs is shewn in Fig. 21, where each 
successive ring is formed by radiations from the margin of the preceding ring. 
These radiations or tufts are occasionally separate, as in the figure, but generally in 
optical contact, so as to form a luminous ring in which the tints are not uniform. 
In weak solutions of mannite, the crystallisations are exceedingly delicate, 
and the light which they polarise scarcely visible. 
18. Oxalurate of Ammonia (pure.)\—This salt, to which my attention was 
called by Professor Grecory, and which, according to that chemist, is probably 
identical with the Lithoxanthate of Ammonia, gives very beautiful negative cir- 
cular crystals. 
With weak aqueous solutions the discs are small and beautiful, and very much 
like those from Lithoxanthate of Ammonia, the cross sometimes consisting wholly 
of circular discs, and at other times of a few discs interspersed among dendritic 
crystallisations. 
From strong solutions the discs are often nearly opaque, and round them are 
formed concentric rings, consisting of marginal radiations, as in Fig. 20, their 
elements being often in optical contact, and yielding different polarised tints. 
Occasionally we find discs, sometimes large ones, in which the central circle con- 
sists of tints of the green of the second order, with a feebly-developed black cross, 
descending to the white of the first order. This is followed by a narrow black 
concentric space, beyond which the mite tint reappears, and rises to the yellow of 
the second order, which again descends to white, thus completing the second ring. 
