XXI. 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. By David Brewster, LL. D. F. R. S. Edin. F. A. S. E. in a letter to the Right Hon. Sir Joseph Banks, Bart. K. B. P. R. S. &c. &c. &c

Abstract

Dear Sir, The 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 results themselves will be deemed of sufficient importance to justify me in submitting them so early to your consideration. During an extensive course of experiments on the depolarisation 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 depolarisation of the transmitted light, which gradually went off as the heat diminished. This effect I at first ascribed to an incipient crystallisation in the included substance; but the increase of the quantity of depolarised light by an augmentation of temperature refuted this conjecture, and rendered it necessary to search for some other cause.