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Proceedings of the British Association: — 325 
glass from St. Andrews, a change of a vety didefentipaa is ef- 
fected. In some cases the siliceous and the metallic elements of 
the glass are separated in a very singular manner, the particles of 
silex having released themselves from the state of constraint pro- 
duced by fusion and subsequent cooling, and arranged themselves 
circularly round the centre of decomposition; while the metallic 
particles, which are opake, have done the same thing in circles 
alternating with the circles of the siliceous particles. This resto- 
ration of the silex to its crystalline state, is proved by its giving 
the colors of polarized light, and possessing an axis of double re- 
fraction. The most valuable glass articles manufactured by 
Fraiinhofer, of Munich, seemed to be peculiarly liable to some su- 
perficial decomposition of this kind. A prism of this glass in the 
Observatory of Paris had become absolutely black. A prism be- 
_ longing to himself had become quite blue on the surface, although 
as yet its action on light was not affected. The largest object 
glass of the principal telescope in the Observatory of Edinburgh 
had begun to show decided symptoms of superficial decomposi- 
tion, and many other instances could also be mentioned. M. La- 
mont, professor of astronomy at Munich, who is in the constant 
habit of using Fraiinhofer’s glasses, stated that there is an easy 
and effectual remedy for this tendency of Fraimhofer’s glass to 
deteriorate on the surface, which was to rub it frequently with 
the finer parts of whiting, prepared by elaborating a mass of whi- 
ting in water, the fine powder to be dried and used on old soft 
inehh 
On the rings of Polarized Light produced ¢ in specimens of de- 
composed glass ; by Sir D. Brewster.—In the course of a series 
of experiments on the connexion between the absorption of light 
and the colors of thin plates, published in the Phil. Trans. 1837, 
Laccidentally observed under the polarizing microscope, certain 
phenomena of polarized tints of great beauty and singularity. 
These tints were sometimes linear and sometimes circular, and in 
Some specimens they formed beautiful circular rings traversed by 
a black cross, resembling the phenomena of’ mineral crystals, or 
those produced by rapidly cooled circular plates or cylinders of 
_ glass. Having found in the decomposed glass from St. Andrews; 
that the siliceous particles had resumed their position as regular 
crystals, and arranged themselves circularly round the centre of 
decomposition, I was led to suppose = this was the cause of the 
Vol. xz, No. 2,—Jan.-March, 1841. 
