438 Brewster‘’s Experiments on the Properties 
lidation of the outer crust, the interior part of the drop had a 
structure similar to that of fluid glass, or at least, that the 
ultimate particles were in both cases at the same distance, 
having been prevented, in the ease of the drop, from approach- 
ing each other by the action of the external coat. 
I therefore procured several of these drops made of bottle 
glass, and upon exposing them to a polarised pencil, I found 
that they not only depolarised it completely, but produced the 
alternation of the prismatic colours, which I was so anxious to 
discover. In order to observe this interesting phenomenon 
with more satisfaction, I obtained several drops made of flint 
glass. All of them exhibited the same phenomena which 
appeared in those made of bottled glass, and there was a 
decided approximation to neutral axes in lines parallel and 
perpendicular to the axis of the drop. When the polarised ray 
was transmitted through a part of the drop nearer to the 
slender stem, the neutral and depolarising axes became still 
more distinct, till at a certain thickness of the stem these axes 
were as completely developed as in the most perfect crystals. 
Having succeeded in detaching the thick bulb of the drop 
from its long slender tail, I ground upon it two parallel sur- 
faces, perpendicular to the axis of the drop, and other two 
parallel to the same axis. When polarised light was trans- 
mitted in both these directions, it w^as depolarised as before, 
and exhibited the segments of the coloured rings, but there 
was no appearance of any neutral axes, the depolarisation 
being equally complete in every position of the drop. 
When a piece of plate glass is brought to a red heat, and 
suddenly cooled by immersion in w^ater, it is intersected by 
numerous fissures, arranged somewhat like the cells of a 
