834 Dr Brewster on the Optical PrcypeHies of Amber. 
the polarising structure could be altered by changing their 
forms, though not to the same degree as in plates of crystallised 
glass. In. some specimens, a negative structure is developed 
round the perforation, as in tubes of glass 
5. Along the axis of a fine stalactite of amber containing in- 
sects, I discovered that remarkable structure which occurs in 
quartz^ and produces the phenomena of circular polarisation, 
the tints descending in the scale as the analysing prism was 
turned from right to left. ' This stalactite was 1 1 inches long, 
and about 1 inch in diameter, and did not polarise the usual 
tints beyond the yellow of the first order. 
6. Some specimens of amber did not possess the polarising 
structure in the slightest degree, and in this respect resembled 
Gum Copal, Gum Galhanum, Gum Juniper, and Gum Mastic. 
All specimens of this kind must have been indurated very slow- 
ly, and in such an uniform manner that the induration went on 
with equal rapidity in the interior as at the exterior surface of 
the mass. 
7. Every specimen of amber is capable of having its polaris- 
ing structure altered, or a new polarising structure communi- 
cated to it, by heat, or by mechanical compression or dilatation. 
8. In pieces of amber filled with globules of air, a polarising 
structure indicated by four minute sectors of polarised light is 
developed round the globules, by the pressure arising from 
the expansive force of the included air. 
These results, obtained from a very extensive examination of 
,speoimens, appear to establish beyond a doubt, that Amber is an 
indurated vegetable juice, eaid that the traces of a regular struc- 
ture, indicated by its action upon polarised light, are not the 
effect of the ordinary laws of crystallisation by which Mellite has 
been formed, but are produced by the same causes which influ- 
ence the mechanical condition of Gum Arabic, and other gums 
which are known to be formed by the successive deposition and 
induration of vegetable fluids. 
Edinburgh, Feb. 18. 1820. 
J^ee Ediru Trans, vol. yiii. p. 362. 
