Dr, Brewster's Experiments, &e, 4,37 
the quantity of depolarised light by an augmentation of tem- 
perature refuted this conjecture, and rendered it necessary to 
search for some other cause. 
Having procured a plate of glass about ^ of an inch thick, 
I brought it- nearly to the temperature of a red heat. When 
a pencil of light polarised by reflection was transmitted in a 
perpendicular direction through the heated glass, the whole 
of the light was completely depolarised, and found a ready- 
passage through a plate of agate, having its veins perpendi- 
cular to the plane of reflection. As the temperature diminished, 
the quantity of depolarised light seemed to diminish in the 
same proportion ; and after the glass had completely cooled, 
the whole of the pencil preserved its original polarisation, and 
refused to penetrate the agate. Hence it follows that glass 
brought to a certain temperatur'e forms two images, and polarises 
them in an opposite manner, like all doubly refracting crystals, the 
one image being coincident with the other. 
The analogy thus indicated between heated glass and 
regularly crystallised bodies was in some degree imperfect, 
as I could perceive no traces of the coloured rings which almost 
every crystal exhibits by polarised light. It occurred to me, 
that the temperature of the glass might not have been suffi- 
ciently high, and that the phenomenon of the coloured rings 
could only be developed when the glass was in absolute fusion, 
or in a state approaching to it. The impracticability, however, 
of making any experiments of this kind with melted glass, 
compelled me to have recourse to another expedient. 
In the formation of glass tears, or Rupert's drops, as they 
are sometimes called, by dropping melted glass into cold water, 
it appeared probable, that in consequence of the sudden conso- 
MDCCCXIV. 3 L 
