regularly crystallized Bodies. 143 



^ver perpendicular to the axis of a prism of rock crystal, 

 (the axis of double refraction) is sensibly the same. 



% All planes parallel to the axis do not possess the same elas- 

 tic state ; but if we take any three of these planes, so that the 

 angles which they form with one another are equal, their elas- 

 tic state is the same. 



With regard to light, all the planes parallel to the axis possess 

 exactly the same properties, so that the information respecting 

 structure thus given by sonorous vibrations, is not of the same or- 

 der as that given by means of light. M. Savart is of opinion that 

 the first method indicates more particularly the elastic state, and 

 the force of cohesion in the different directions of all the planes 

 of the integrant particles, whilst the phenomena of light, de- 

 pending more especially on the form of the particles, and the 

 position which they affect round their centre of gravity, they 

 are to a certain point independent of the mode of junction of 

 the different laminae of which the crystal is formed. 



3. The transformations of the nodal lines of a series of plates 

 cut round one of the edges of the base of the prism, are quite 

 analogous to those which take place in a series of plates cut 

 round the intermediate axis in bodies which possess three un- 

 equal and rectangular axes of elasticity. 



4. The transformations in a series of plates perpendicular to 

 £iny one of the three planes which pass through the opposite 

 edges of the hexaedron, are in general analogous to those of a 

 series of plates cut round a Hne, which divides into two equal 

 parts the plane angle contained between two of the three axes 

 of elasticity, in bodies where these axes are unequal and rec- 

 tangular. 



^. By means of the acoustic figures on a plate cut nearly paral- 

 \ lei to the axis, and not parallel to two faces of the hexaedron, 

 we may always distinguish which are the faces of the pyramid 

 which are susceptible of cleavage. We may also obtain the 

 same results by the arrangement of the modes of division of a 

 plate cut nearly parallel to one of the faces of the pyramid. 



6. Whatever be the direction of the plates, the optic axis or 

 its projection on their plane, always occupies there a position 

 which is closely connected with the arrangement of the acoustic 

 lines. For example, in all the plates cut round one of the 



