Page:Scientific Memoirs, Vol. 1 (1837).djvu/267



OCK Crystal most ordinarily occurs under the form of a hexahedral prism, terminated by pyramids with six faces (fig. 1. pl. IV.). Although this substance does not admit of cleavage by the ordinary means, it is assumed, from analogy, that its primitive form is a rhombohedron, like that which would be obtained if the crystal were susceptible of cleavage parallel to the three non-adjacent faces of the pyramid, such, for example, as $$a X b$$, $$e X f$$, $$cXd$$, and their parallels $$a^\prime Y b^\prime$$, $$e^\prime Y f^\prime$$, $$c^\prime Y d^\prime$$. The accuracy of this induction is besides confirmed by a very simple experiment, which consists in making a prism of rock crystal red hot, and suddenly cooling it; an operation which determines its fracture, and which most frequently, gives as the result pieces of crystal which have the form of rhombohedrons.

Setting out with these notions with which mineralogy furnishes us, it is obvious that circular plates taken parallel or perpendicular to the axis, parallel to a face of cleavage or of non-cleavage of the pyramid, &c. ought to present different phænomena with respect to sonorous vibrations, since the cohesion and elasticity are not the same in these different directions. Consequently, to simplify as much as possible the examination of these phænomena, we have had cut, from different pieces of rock crystal, a considerable number of circular plates, at first taken in different azimuths of a plane perpendicular to the axis, fig. 2. and fig. 2, bis; then, according to the azimuths of a plane perpendicular to two parallel faces of the hexahedron, and passing through its axis, fig. 3. and fig. 3, bis; lastly, according to the different azimuths of a plane passing through the axis and two opposite edges of the crystal fig. 4. and 4 bis.

As it was necessary to support this general disposition of the experiments by facts, it was indispensable to ascertain first, that the elastic state of the crystal is the same for all the planes parallel to the natural faces of the hexahedron, and next, that it is also the same for all the