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 applied. 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 axis of di- 

 latation, is negative, and consequently the polarising force pro- 

 duced by dilatation is positive, like that of zircon, &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 



