PLUATE OF LIME, AND THE DIAMOND. 163 



*' cristaux qui par leur nature ne possedent point la double re- 

 " fraction. Enfin ce qui acheve de confirmer cette man fere 

 " de voir, M. Brewster a encore imprime les m£mes proprie- 

 " tes a des plaques de ge!6e animale, en exercant sur elles ex- 

 " terieurement une pression passagere ', de sorte que les cou- 

 " leurs paraissent tant que la pression dure, varient avec elle, 

 " et evanouissent quand elle cesse." 



Had M. Biot repeated the experiments to which he alludes 

 in the preceding passage, and compared the results with those 

 produced by heat and rapid evaporation, he would have in- 

 stantly seen that the two classes of phenomena are essentially 

 distinct, and could not possibly have the same origin. In the 

 polarising structure produced by heat, by rapid cooling, and by 

 evaporation, the axes are constantly related to the edges, the 

 angles and the surfaces of the bodies which are employed, and 

 the nature and form of the optical figure which they exhibit, 

 depend solely on the outline and on the thickness of the mass. 

 In cubical and octahedral crystals, on the contrary, the polarising 

 axes are related to the axes of the crystals themselves, and have 

 no connection whatever with the shape or outline of the mineral. 

 If we take a cube of muriate of soda, for example, which has 

 not sufficient thickness for developing its structure, and expose 

 it either to heat or pressure, we shall find it impossible to ap- 

 ply either of these powers in such a manner as to produce a 

 crystallisation that has the smallest resemblance to the effect 

 shewn in Plate IV. fig. 4. If the muriate of soda is thick 

 enough to render its structure visible, then the crystallisation 

 superinduced by heat or pressure may be seen at the same 

 time with its own natural crystallization. The results obtained 

 by means of the diamond, as shewn in fig. 6. and with which 

 M. Biot was not acquainted when he wrote the preceding pas- 



X 2 sage, 



