
SIR DAVID BREWSTER ON CIRCULAR CRYSTALS. 609 
Borax. I was thus enabled to study the phenomena which they exhibit in their 
formation, their structure, and their subsequent decomposition. 
In the expectation of obtaining a greater variety of structure, and discovering 
new phenomena, I submitted to examination about 300 doubly-refracting sub- 
stances, and among these I discovered nearly seventy that give circular crystals, 
about thirty of which are positive, like Zircon, and forty negative, like Calcareous 
spar. 
In the course of these experiments, which have occupied much of my time, I 
have observed many new and splendid phenomena, which lay open an extensive 
field of research, and promise to throw much light on those abnormal crystallisa- 
tions which take place under the constraining influences of heat and pressure, 
and also on their subsequent decomposition and return to their molecular state. 
In submitting to the Society an account of these experiments, I shall begin 
with the Lithoxanthate of Ammonia, as it exhibits a greater variety of phenomena, 
and is more easily converted into circular crystals than any other salts with 
which I am acquainted. 
1. Lithoxanthate of Ammonia.—This substance, under ordinary circumstances, 
crystallises in minute prisms, often in beautiful dendritic forms, and in spherical 
groups of crystals in which the prisms are not in optical contact, and yet suffi- 
ciently united to exhibit the black cross at the centre of the sphere. 
When the circular crystals are produced, and are transparent, they have very 
different aspects in different specimens. In their simplest form, they are united 
in a continuous film, each circular crystal exhibiting four luminous sectors sepa- 
rated by a black cross, the arms of which are, of course, always parallel and 
perpendicular to the plane of primitive polarisation. The light polarised by the 
sector is the blwe of the first order, often rising to the white, and sometimes to the 
yellow, of the same order. 
When we look at a small and bright luminous disc through a film of such 
crystals, we see a halo, and sometimes two halos, the diameter of the halo dimi- 
nishing as the circular crystals increase in size. When the film is placed in the 
polariscope, the halo is converted into four luminous sectors, and into eight when 
it is double, exactly the same as those produced by oil of mace, and shewn in 
Figs. 1 and 2. 
When the circular crystals are separate, their structure is more complex, and 
their appearance more beautiful. In one of these, shewn in Fig. 3, I have ob- 
served, but only once, the three first orders of colours of thin plates, exactly like 
the uniaxal system of rings in regular crystals; and consequently, the thick- 
ness of the spicular crystals which composed them must have increased from the 
centre outwards, according to the law in Newron’s Table of Periodical Colours. 
This result was so remarkable, that I determined the character of the three 
