439 
impressed upon Light by heated Glass, 
honeycomb ; but as the rupture of the surface leaves the par- 
ticles at liberty to dispose themselves at distances correspond- 
ing to the temperature, the glass exhibits no power either in 
polarising or depolarising light. 
The effects of heat upon crystallised bodies, but particularly 
upon those which, like fluor spar and muriate of soda, do not 
possess the property either of polarisation or depolarisation, I 
propose immediately to investigate ; but even if the inquiry 
should be carried no farther, the results which I have described 
must be considered as of the utmost importance, not only as 
giving us positive information respecting the structure of cry- 
stallised bodies, but as opening a new path to the solution of 
the great problem of double refraction. 
I have the honour to be, 
dear Sir, 
your most obedient and humble servant, 
DAVID BREWSTER. 
Edinburgh, April 8th, 1814, 
To the Right Hon. Sir Joseph Banks, Bart. K. B. P.R.S. 
&c. &c. See, 
3 L 2 
