7 s Dr. Brewster on new properties of heat , 
flew off with violence from the lower one, and a black fringe 
instantly sprung up below the new edge mp, just as if the 
upper part of the glass had never been in contact with the 
lower part. In another experiment, attended with the same 
result, the crystalline structure above m n instantly vanished 
when the crack reached o , although the two pieces of glass 
still cohered with some force. When the fissure m n was 
placed vertically, as in Fig. 16. (PI. III.) the same effect took 
place as if the two pieces had been separate, and no change 
was observed by cementing them with Canada balsam. 
Instead of fissures, I now substituted deep grooves cut across 
the glass. A thick plate which had a horizontal groove cut 
half through it, and extending from edge to edge, was laid 
upon the hot iron. The white fringes appeared imperfectly 
above the groove, and an undefined dark wave below it, as if 
some fluid had been obstructed in its passage through a nar- 
row channel. It is not improbable that this dark wave was 
occasioned by the combination of two white fringes of different 
sets. For if BC, DE, Fig. 17. (PI. III.) be a vertical section 
of the plate, and AFG the groove, the parts GF omE may 
be considered as acting like a separate plate, and will there- 
fore have op, rn n for its black spaces, while the other part, 
DBCAF 0 m, will also act as a separate plate, and have tu,r s, 
for its black spaces. But the white of the exterior fringe of 
the first of these plates between FG and op will thus be oppo- 
site to the white of one of the interior sets in the other plate, 
and as these are produced by opposite crystallizations, a black 
tint will be the result of their union. The bursting of the 
plate in the direction of the groove, prevented any farther 
examination of the phenomena. 
