144 M. Savart on the Elasticity of 



edges of the base of the prism, the optic axis or its projection 

 corresponds constantly with one of the two straight lines 

 which compose the nodal system formed of two lines at right 

 angles to each other. 



Though there is a great analogy between these phenome- 

 na and those observed in wood, yet rock crystal cannot be 

 numbered among bodies which have three rectangular and un- 

 equal axes of elasticity, and still less in the number of those, 

 all of whose parts are symmetrically arranged round a single 

 straight line. The same phenomena, indeed, are constantly re- 

 produced in three different positions, and it would appear that 

 every thing is related to the different directions of cleavage — 

 to the faces and to the edges of the primitive rhombohedron. 

 Thus all the plates cut parallel to the natural faces of the 

 hexaedron enjoy exactly the same properties, and these pro- 

 perties are very different from those of plates equally parallel 

 to the axis, but which are perpendicular to two opposite faces 

 of the octohedron. Those plates, too, which are parallel to the 

 cleavable faces of the pyramid, emit the same sounds, and pro- 

 duce the same acoustic figures ; whilst plates parallel to the 

 three other faces present figures different from those of the 

 preceding. It would appear, therefore, to follow from this 

 identity of phenomena for three distinct positions, that there 

 is in rock cr)stal three systems of axes or principal lines of 

 elasticity. 



By comparing the phenomena in rock crystal with those in 

 wood, M. Savart concludes, that the individual axes of each 

 of these three systems are as follows ; The shorter diagonal of 

 each face of the primitive rhombohedron is the axis of greatest 

 elasticity for each system, while the great diagonal of the face 

 of the rhombohedron is the intermediate axis of elasticity. 

 The axis of least elasticity is perpendicular to the axis of inter- 

 mediate elasticity, and is inclined 57° 40' 13" to the axis of 

 greatest elasticity, this angle being the inclination of the rhom- 

 bohedral face upon the plane passing through the great dia- 

 gonal of the same face. 



" The transparent carbonate of lime, and the carbonate of 

 lime and iron appear to possess elastic properties, in general 

 analogous to those of rock crystal. Like it, they possess three 



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