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XXI. Results of some recent Experiments on the Properties im^ 
pressed upon Light by the Action of Glass raised to different 
Temperatures, and cooled under different Circumstances. By 
David Brewster, LL. D. F. R. S, Edin. and F. A. S. E. in a* 
Letter to the Right Hon. Sir Joseph Banks, Bart, K. B, 
P. R. S. &c. &c. &c. 
Read May 19, 1814,. 
Dear Sir, 
fnE interest which you take in every thing which contributes 
to the progress of science, renders it unnecessary to make any 
apology for communicating to you the results of some recent 
experiments on the properties impressed upon light by the 
action of glass, raised to different temperatures and cooled 
under different circumstances. The imperfect state of these 
experiments will still require your indulgence ; but I trust that 
independent of all farther developement, the resulp themselves 
will be deemed of sufficient importance to justify me in sub- 
mitting them so early to your consideration. 
During an extensive course of experiments on the depo- 
larisation of light, by soft bodies melted and cooled between 
plates of glass, it was necessary to bring the plates to different 
temperatures, in order to fuse the substance which they 
inclosed. When the body melted at a high temperature and 
was in a fluid state, I frequently perceived a partial depolari- 
sation of the transmitted light, which gradually went ofl'as the 
heat diminished. This effect I at first ascribed to an incipient 
crystallisation in the included substance; but the increase of 
