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XXXVIII.-~—On the Production of Crystalline Structure in Crystallised Pomders, by 
Compression and Traction. By Sir Davip Brewsver, K.H., D.C.L., F.R.S., 
V.P.R.S. Edin., and Associate of the Institute of France. 
(Read 7th March 1853.) 
The influence of compression and dilatation in producing the doubly refract- 
ing structure in solids of all kinds, whether crystallised or uncrystallised, which 
do not possess it, and in modifying that structure in all crystals which do possess 
it, has been long known; ‘but with this class of phenomena, those which I am 
about to describe have no connection whatever. 
In the course of experiments on the double reflexion and polarisation of light 
which I discovered in the chrysammates of potash and magnesia, murexide, and 
other crystals, I was surprised to find that these substances could be spread out 
upon glass by hard pressure, like grease or soft wax, and that in the case of chry- 
sammate of potash and other bodies, when the powder could scarcely be distin- 
guished from snuff, I obtained a transparent film, exhibiting the phenomena of 
double reflexion and polarisation from its surface, as perfectly as if I had been 
using a large crystal. 
In subsequently repeating these experiments, and examining, under polarised 
light, the film thus produced by compression and traction, I was surprised to ob- 
serve that the streaks and separate lines of the film, as well as the film itself, had 
regular axes of double refraction, as if they were regularly crystallised portions of 
the substance under examination. These streaks and capillary lines, which were 
often of extreme minuteness, did not appear to consist of insulated particles merely 
dragged into a line, but when the substance possessed the new property in per- 
fection, the lines of polarised light were continuous, and the crystallographic as 
well as the optical axes of the particles were placed in that lie. In other cases, 
where the experiment was less successful, the insulation of the particles was 
easily recognised, though the general mass of them was crystallographically 
arranged. 
In making these experiments, the natural crystalline powder, or the particles 
of the crushed crystal, may be placed, either upon a polished glass surface, or upon 
a piece of glass ground on one side. In those cases where the substance is soft, the 
polished surface is preferable, but when the powder is hard and considerable pres- 
sure necessary, it is better to place it upon the ground surface of a piece of glass, 
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