9.6 2 ' Dr. Brewster on the laws of polarisation, &c. 
and the artificial crystals is still more striking than the 
agreement of the laws of the tints, as all the different classes 
of crystals can be imitated in glass. This similarity will 
appear from the following comparison : 
Artificial crystals. 
i A circular plate of glass 
crystallized by the trans- 
mission of heat, 
a A circular plate of glass 
heated, and in the act of 
cooling. 
3 A rectangular plate of glass 
when heat is in the act of 
being transmitted through 
the plate. 
4 A rectangular plate of glass 
heated, uniformly, and in 
the act of cooling. 
5 A circular plate of glass, 
permanently crystallized to 
a slight degree, and then 
heated, so as to produce, in 
the act of cooling, a tint 
Real crystals. 
1 Crystals of zircon, quartz, 
ice, &c. 
2 Crystals of calcareous spar, 
beryl, ruby, &c. 
3 Crystals of topaz, sulphate 
of lime, &c. in which the 
principal force is attrac- 
tive. 
4 Crystals of mica, nitre, & c. 
in which the principal 
force is repulsive. 
5 Cubical, octohedral, and 
rhomboido-dodecahedral 
crystals, in which the po- 
larising force is destroyed 
by opposite actions. 
equal to its permanent tint. 
Although these artificial crystals exhibit in the most per- 
fect manner all the phenomena of fixed and moveable pola- 
risation, and develope the tints of Newton's scale with much 
more regularity than even sulphate of lime, yet M. Biot has 
described the phenomena which they exhibit under the head 
of imperfect crystallization. If such a distinction were neces- 
