8 s Dr. Brewster on new properties of heat, 
I 6 Cl ^ .iy ■' ; (J *• <** -tf. V' ' ? •• : • ■■ ' X J ' '■■ ' . \ \ f Ti£ ' J 
Proposition XXV. 
If a plate of crystallized glass is cut in two pieces by a diamond 
along the line a b, Fig. 2 o. (PI. III. ) each of the separate plates 
will exhibit the properties of a whole crystallized plate. The 
portion r sop of the separate plate which had formerly the struc- 
ture of the attractive class of doubly refracting crystals , has now 
the structure of the repulsive class ; another portion op which 
had the attractive structure , has now an intermediate structure , 
similar to that of muriate of soda, &c* and so on with the other 
parts of the crystal. 
If a plate of crystallized glass ABCD, Fig. 20. ( PL III . ) is 
cut with a diamond along the line a b, through the central 
white fringe, the portion a b DC has the same structure as the 
whole plate, as is represented at rsGH, Fig. 21. (PI. III.) 
a dark space having started up at op, while the other dark 
space MN has descended to«»; the portion r sp 0, m n GH, 
have now the structure of the repulsive class, and the 
intermediate portion op n m-, that of the attractive class of 
crystals. 
The same change takes place in the upper plate A B ba. 
Fig. 20. (Pi. III.) which has the appearance shown at EF sr 
Fig. 21. (PI. III.) In one case I found that the fringes in the 
upper plate were exactly the reverse of those in the under 
plate. 
When the plate is cut perpendicularly to the fringes, an 
analogous effect is produced. Terminal fringes instantly ap- 
* See the Transactions of the Royal Society of Edinburgh, Vol. VIII. Part I. 
where the properties of this intermediate class of crystals are described. 
