as exhibited in its propagation along plates of glass. 83 
pear at the new extremities. A similar, though a more unex- 
pected result, was obtained by breaking in pieces a large 
plate, in which the crystallization was extremely irregular, 
polarising here and there a portion of white light. The plate 
had a small crack in it, and when broken in three pieces, 
principally along a line nearly parallel to its edge, each piece 
was regularly crystallized, having the two black spaces with 
their accompanying fringes of white light. 
The same effects are produced when the plate is cut in 
pieces by a slitting wheel, or has its shape altered by grinding. 
The preceding experiments are not easily made, as it is 
very difficult to cut this kind of glass with a diamond. It 
generally flies into many pieces as soon as it is scratched, and, 
when this does not happen, the pieces separate of their own 
accord, some time after the diamond has been applied. 
Scholium. 
The truth contained in the preceding Proposition is analo- 
gous to the celebrated experiment in magnetism, where the 
smallest portion detached from the extremity of a magnet, 
becomes itself a complete magnet, possessing distinct north 
and south poles. The exhibition of the same phenomena in 
glass transiently crystallized during the propagation of heat, 
as described in Prop. XIII., might have been supposed to arise 
from some new property of heat, which enabled it to act on 
the remote edge of the glass without any sensible indication 
of its presence. This opinion, however, is to a certain extent 
excluded by the results obtained with glass permanently crys~ 
tallized and having an uniform temperature. Any portion of 
the glass passes with the utmost facility from one crystalline 
M 2 
