546 Royal Society. 
other with a force greater than that of the lamine of sulphate of lime, 
or of mica ; but less than those of calcareous spar. When the ad- 
hering plates are separated, the internal surfaces are sometimes co- 
lourless, especially when these surfaces are corrugated or uneven ; 
but they are almost always covered with an iridescent film of the 
most brilliant and generally uniform tint, which exhibits all the va- 
riety of colours displayed by thin plates or polarizing lamine. This 
substance, like most crystallized bodies, possesses the property of 
refracting light doubly ; and, as in agate and mother-of-pearl, one of 
the two images is perfectly distinct, while the other contains a con- 
siderable portion of nebulous light, varying with the thickness of the 
plate, and the inclination of the refracted ray. Like calcareous spar, 
it has one axis of double refraction, which is negative ; and it gives, 
by polarized light, a beautiful system of coloured rings. It belongs 
to the rhombohedral system, and, as in the Chaux carbonatée basée of 
Haiiy, the axis of the rhombohedron, or that of double refraction, is 
perpendicular to the surface of the thin plates. As mother-of-pearl 
has, like arragonite, two axes of double refraction; this new sub- 
stance may be regarded as having the same optical relation to cal- 
careous spar that mother-of-pearl has to arragonite. 
The flame of a candle, viewed through a plate of this substance, 
presents two kinds of images; the one bright and distinct, the others 
faint and nebulous, and having curvatures, which vary as the incli- 
nation of the plate is changed: the two kinds being constituted by 
oppositely polarized pencils of light. On investigating the cause of 
these phenomena, Sir David Brewster discovered it to be the imper- 
fect crystallization of the substance ; whence the doubly refracting 
force separates the incident light into two oppositely polarized pen- 
cils, which are not perfectly equal and similar. In this respect, in- 
deed, it resembles agate, mother-of-pearl, and some other substances ; 
but it differs from all other bodies in possessing the extraordinary 
system of composite crystallization, in which an infinite number of 
crystals are disseminated equally in every possible azimuth, through 
a large crystalline plate; having their axes all inclined at the same 
angle to that of the larger plate, and producing similar phenomena 
in every direction, and through every portion of the plate: or this re- 
markable structure may be otherwise described, by saying that the 
minute elementary crystals form the surfaces of an infinite number of 
cones, whose axes pass perpendicularly through every part of the 
larger plate. 
An examination of the phenomena of iridescence afforded by this 
new substance, leads him to the conclusion that the iridescent films 
are formed at those times when the dash-wheel is at rest, during the 
night, and that they differ in their nature from the rest of the sub- 
stance. These phenomena illustrate in a striking manner some ana- 
logous appearances of incommunicable colours presented by mother- 
of-pearl, which had hitherto baffled all previous attempts to explain 
them ; but which now appear to be produced by occasional intermis- 
sions in the processs by which the material of the shell is secreted 
and deposited in the progress of its formation. 
