284 ON THE EFFECTS OF PRESSURE IN ALTERING THE 
the specimen, in the same manner as in plates of glass. Hence 
it follows, that since the polarising force of crystals is in no re- 
spect influenced by their external shape, a new and moveable 
polarising force is generated by pressure, which increases or di- 
minishes the effect of the permanent force, according to the di- 
rection in which it is anplied. The effect, therefore, of a cry- 
stallised plate subjected to pressure, is the same as if we com- 
bined it, when free from pressure, with a similar plate of the 
same substance, destitute of any polarising force, and pressed 
in a similar manner. The force residing in the ultimate par- 
ticles of the crystal is unchangeable, and the pressure deve- 
lopes the new force, by merely altering their state of aggrega- 
tion. 
With the aid of these views, we may now predict all the 
changes which can be produced upon positive and negative 
crystals, by mechanical compression and dilatation. 
When the two parallel surfaces of a transparent solid are 
brought nearer each other by pressure, the tint, in the direction 
of a line perpendicular to these surfaces, which may be called 
the axis of compression, is negative, and therefore the polarising 
force produced by compression is negative, like that of calcare- 
ous spar, &c. When the two surfaces are, on the other hand, 
separated from each other by dilatation, the tint in the direc- 
tion of the perpendicular, which may be called the awis of di- 
latation, is negative, and consequently the polarising force pro- 
duced by dilatation is positive, like that of ztrcon, &c. 
Hence, if we take plates of crystals, and apply the forces to 
parallel surfaces, we shall obtain the results contained in the 
following Table : 
- TABLE 
a 
