264 Dr. Brewster on the taws of polarisation, &c . 
its form is rectangular, or elliptical, it has two axes of pola- 
risation, the strongest of which appears to be positive, and 
the weakest negative. 
In crystals, on the contrary, the polarising structure is 
wholly independent of their outward form, and of any varia- 
tion of density in the aggregation of their particles, and must 
therefore depend on the form of their integrant molecules, 
and the variation in their density. Is it not more than pro- 
bable, therefore, that we have in our artificial crystals, a 
representation of the ultimate particles of crystallized bodies ? 
When these crystals have a spherical form, diminishing in 
density towards an axis, and have these axes arranged 
by the laws of crystallization, they will constitute a 
crystal of the positive class, like ice, zircon , quartz , &e. 
and the only difference between ice and water will be, 
that in the ice the particles have their axes regularly 
disposed ; while in the water, the axes have every pos- 
sible direction, so as to create a general equilibrium of the 
polarising forces. When the density of the spheres increases 
towards their axes, their symmetrical combination will con- 
stitute a crystal of the negative class, such as beryl, calcareous, 
spar. Sc c. 
But if the particles, instead of being spheres, are either 
prolate or oblate spheroids, having their polarising axis at 
right angles to the axis of revolution ; and if their density 
varies, as in elliptical plates of glass, along which heat is in 
the act of being transmitted, they will constitute by their 
regular combination, the class of crystals with two axes, such 
as topaz, sulphate of lime. See . in which the positive force pre- 
dominates. If the density, on the other hand, varies as in 
