refraction to glass, &c. by pressure \ 167 
increase in number and encroach on the interior fringes ; but on 
the lower or convex side, the fringes diminish in number, and 
are encroached upon by the interior fringes. Uncrystallized 
plates, when compressed or dilated, exhibit similar effects if 
combined with crystallized plates not subjected to compression 
or dilatation. 
Proposition XII. 
Muriate of soda, f nor spar , diamond , obsidian , semi-opal, horn , 
tortoise-shell, amber, gum copal , caoutchouc, rosin, phosphorus, 
the indurated ligament of the chama giganlea, and other sub- 
stances , that have not the property of double ref r action , or that 
have it in an imperfect manner fare capable of receiving it by 
compression or dilatation. 
Of all the substances mentioned in the Proposition, obsidian, 
muriate of soda, and gum copal, receive from pressure the 
greatest polarising force. Gum copal, in particular, exhibited 
a greater number of fringes than a piece of glass subjected 
to the same pressure. 
Proposition XIIL 
Calcareous spar, rock crystal, topaz, beryl, and other minerals that 
already possess in a high degree the doubly refracting structure, 
suffer no change by compression or dilatation. 
The state of compression or dilatation in which the particles 
of these crystals are already placed, according to the class in 
which they belong, is so great, as not to experience any 
change from the application of ordinary forces. I have 
* See the Edinburgh Transactions, Vol. VIII. Part I. where I have shown that 
diamond, muriate of soda, &c. possess, imperfectly, the structure of both classes of 
doubly refracting crystals. 
