COLOURS OF CRYSTALLINE PLATES. 147 



yaries with the shape of the mass.* The laws of these pheno- 

 mena have been completely analyzed by Sir David Brewster ; and 

 he has shown that the colours are those of crystallized plates, the 

 direction of the axes, however, being different in different parts of 

 the substance. 



As the double-refracting structure is communicated to bodies 

 which do not possess it naturally, by mechanical compression or 

 unequal temperature, so, by the use of the same means, that 

 structure may be altered in the bodies in which it already resides. 

 Thus Sir David Brewster and M. Biot have found that the double 

 refraction of regular crystals may be altered, and the tints they 

 display made to rise or descend in the scale, by simple pressure. 

 But the changes induced by heat are yet more remarkable. Pro- 

 fessor Mitscherlich discovered the important fact that, in general, 

 heat dilates crystals differently in different directions, and so alters 

 their form ; and their double-refracting properties have been found 

 to undergo a corresponding change. Thus Iceland spar is dilated 

 by heat in the direction of its axis ; while it actually contracts by 

 a small amount in directions perpendicular to it. The angles of 

 the primitive form thus vary, the rhomboid becoming less obtuse,f 

 and approaching the form of the cube. M. Mitscherlich, accord- 

 ingly, conjectured that its double-refracting energy must in these 

 circumstances be diminished ; and the conjecture was fully verified 

 by experiment. This inquiry has been followed up by M. Eud- 

 berg ; and the effects of heat on the refractive indices of double- 

 refracting crystals examined by the direct method of prismatic 

 refraction. In conformity with the observations of M. Mitscher- 

 lich, it was found that the extraordinary index in Iceland spar 

 increased considerably with the temperature, while the ordinary 

 index underwent little or no change. In rock crystal, on the 

 other hand, both indices diminished as the temperature augmented, 

 and nearly by the same amount. In arragonite a similar effect 

 was produced on the three principal refractive indices, the least 



* The experiments of M. Seebeck are recorded in Sehweigger't Journal, 1814. 

 The depolarizing property of unannealed glass seems to have been first noticed by 

 M. Arago ; and was afterwards studied by Sir David Brewster in glass which had 

 been melted and cooled in water. Phil. Trans. 1814. 



t A change of temperature, from the freezing to the boiling-point, produced a 

 <-hange of 8' in the dihedral angles at the extremity of the axis.-2?wtf. Soc. 

 March, 1824. 



L2 



