SECT. ii. TO CRYSTALLINE FORM. 73 



right angles to their former position. Thus the force 

 of heat throws every molecule in the body into corre- 

 lative motion. The angles of all crystals that are not 

 of the octohedral group are changed by heat and vary 

 with the intensity ; the difference between the length of 

 the greatest and least optic axes in such crystals di- 

 minishes as the temperature is raised, increases when 

 it is lowered, and is constant at a given heat. In Ice- 

 land spar heat indirectly affects the doubly refracting 

 power, for the expansion of the crystal in the direction 

 of its axis is accompanied by contraction at right angles 

 to it, which brings the crystal nearer to the cubical 

 form, and consequently diminishes its doubly refracting 

 power. 



According to the researches of M. Angstrom, in 

 crystals with different axes of elasticity the velocity of 

 the molecular vibrations is different in different direc- 

 tions when they are heated. In rock crystal and tour- 

 maline the heat ra liates from a surface cut parallel to 

 the axis of the crystal ; in felspar the radiating surface is 

 at right angles to the symmetrical axis. 



The optical axes of crystals are also affected by 

 pressure. Doubly refracting crystals with one principal 

 axis acquire two when the pressure is perpendicular to 

 it. The new principal axis coincides with the line of 

 pressure or is at right angles to it according as the 

 crystal is positive or negative, that is, according as the 

 extraordinary ray is refracted to or from the optic axis 

 of the crystal. The colours produced by polarization 

 are affected by compression and dilatation according as 

 the crystal is positive or negative. 



Sir David Brewster is of opinion that all the properties 

 of double refraction and the gorgeous phenomena of 

 polarization, whether by crystals or produced in various 

 substances permanently or transiently by heat, cold, 

 rapid cooling, compression, dilatation, and induration, 



