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IV. Ora 72 m properties of heat , as exhibited in its propagation 
along plates of glass. By David Brewster, LL. D. F. R. S. 
Lond. and Edin. In a Letter addressed to the Right Hon. 
Sir Joseph Banks, Bart. G. C. B. P. R. S . 
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Read January 11 , 1816. 
Dear Sir, 
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In two papers published in the Transactions of the Royal 
Society,* I have given some account of the action of heat in 
enabling glass to arrange a beam of light, into two oppo- 
sitely polarised pencils, and I have shown that unannealed 
glass, in the form of Prince Rupert's drops, possesses dis- 
tinct optical axes, and acts upon light like all regularly 
crystallized bodies. 
My attention was sometime ago recalled to this subject, in 
consequence of having discovered that reflection from all the 
metals, and total reflection from the second surfaces of trans- 
parent bodies, produced the same effect as crystallized plates, 
in separating a beam of polarised light into its complementary 
tints. I was thus led to believe, that the existence of two oppo- 
sitely polarised pencils, and the production of the complemen- 
tary colours, were concomitant effects, and I prepared to exa- 
mine the truth of this supposition in the case of heated glass. 
In my early experiments on this subject, I had not observed 
these colours, as I was not then in the possession of a mode 
of detecting them, when they formed the lower tints of the 
* See Phil. Trans. 1814, p. 436, and 1815, p. 1. 
