ioo Dr. Brewster on new properties of heat , 
(PI. V.) the two preceding structures are combined. It has 
three black spaces, mn, \kv, op , the parts D and B have the 
same structure as that which produces the exterior sets of 
fringes, and the parts A,C, the same structure as that which 
produces the interior set in regularly crystallized plates. The 
poles are therefore arranged in the manner shown in Fig. 46. 
(PI. V.) which resembles a magnet with consecutive poles, 
4. Out of nearly one hundred pieces of crystallized glass 
I have found but one which exhibited only two sets of fringes. 
The piece of glass AB, Fig. 47. (PL V.) was intersected in 
cooling with a crack mEn, which extended completely across 
the plate. The parts still cohered with such firmness, as not 
to separate when taken up in the hand. Upon exposing it 
to a polarised ray, it gave two white fringes E, F, separated 
by a dark space OP. The two fringes had opposite charac- 
ters, so that the poles were arranged as in Fig. 48. (PI. V.) 
which resembles that of a perfect magnet. This state of the 
poles, however, is in the case of glass a state of violence, for 
when the plate broke in two pieces at the crack mEn, the 
fringes vanished entirely, and it retained no mark whatever 
of its former crystalline state. The other portion T did not 
act upon polarised light either before or after the separation. 
The pressure of the portion T, therefore, had not allowed the 
other piece of glass to recover from the state of constraint in 
which it was held. 
