ELASTICITY OF REGULARLY CRYSTALLIZED BODIES. 263 



bej'ond which they approach each other until they coalesce in K, after 

 which they again diverge until the plate N, which is parallel to the axis. 

 The nodal system indicated by the dotted lines follows another 

 course ; the summits of the two curves which compose it at first recede 

 from each other, but they soon reapproach each other, and these curves 

 transform themselves into two straight lines in the plate E, where the 

 curves of the other mode of division attain their maximum of recession: 

 beyond this limit they separate, but in a perpendicular direction to 

 that in which they approached, and they attain their inaximum of 

 recession towards the plate H, for which the two systems of curves are 

 nearly similar : they afterwards approach each other, and like those of 

 the other system, they transform themselves, in K, into two straight 

 lines, which intersect each other at riglit angles. Lastly, starting from 

 this point, they diverge again, until the plate N, for which the two 

 systems again become equal, assuming, with respect to the axis of the 

 crystal, a direction different from that which they had taken at I and at 

 H. I must observe that my supply of rock crystal liaving failed at the 

 end of my experiments, I have not been able to cut the plate K ; lut 

 the transformations of the nodal lines so clearly indicate that there 

 ought to be a plate which presents these modes of division, that I have 

 not hesitated to admit its existence. 



The course which the two sounds follow, in this series of plates, is 

 much more simple than that of the nodal figures : at first those of the 

 dotted system become lower, commencing with the plate A, and pro- 

 ceeding as far as the plate E, inclined 51° to the axis, and which gives 

 the sound C like the plate No. 4, inclined the same number of degrees 

 to the axis ; aftem^ards the sound of this system gradually ascends until 

 the plate N parallel to the axis, where it attains its maximum of eleva- 

 tion. As to the sounds of the other series of modes of division, it is 

 observ'ed that they gradually ascend from the plate perj^endicular to the 

 axis unto K, in which the nodal systems both consist of lines crossed 

 rectangularly, and that they afterwards descend again until the plate 

 N parallel to the axis. It is obvious that it is not necessary to examine 

 such plates as A', B', C, D', fig. 4, since they ought to present the 

 same phaenomena as the corresponding plates A, B, C, D : onlj', that 

 which was inclined to the right of the axis in the plates B, C, D is 

 found inclined to the left in the plates B', C, D'. 



lliere is none of the modes of division of this series which is not 

 analogous to some one of those which have been presented to us by 

 bodies in which there are evidently three rectangular axes of elasticity; 

 nevertheless, considered all together, the transformations we have just 

 described present peculiarities which do not exist in the fourth scries of 

 plates of wood, fig. 14, PI. III. The most striking consists in this, that 

 in the transformations of this last series, none of the systems, except 



T 2 



