FLUATE OF LIME, AND THE DIAMOND. 163 
“ cristaux qui par leur nature ne possedent. point la double re- 
“ fraction. Enfin ce qui achéve de confirmer, cette maniére 
“ de voir, M. Brewsrer a encore imprimé les mémes proprie- 
“ tés, des plaques de gelée animale, en exercant sur elles ex- 
terieurement une pression passagere ; de sorte que les cou- 
“ leurs paraissent tant que la pr Pani Gms 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 
al 
i 4 
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. Bior was not acquainted when he wrote the preceding pas- 
X 2 : sage, 
