264 FELIX SAVART’S RESEARCHES ON THE 
the first and the last, was rectangular, whilst in rock crystal this mode 
of division may establish itself. 
Summary. 
Ist. The elasticity of all the diameters of any plane perpendicular to 
the axis of a prism of rock crystal, may be considered as being sensibly 
the same. 
9nd. All the planes parallel to the axis are far from possessing the 
same elastic state ; but if any three of these planes be taken, restricting 
ourselves only to this condition, that the angles which they form with 
each other are equal, then their elastic state is the same. 
3rd. The transformations of the nodal lines of a series of plates cut 
round one of the edges of the base of the prism are perfectly analo- 
gous to those which are observed in a series of plates cut round the 
intermediate axis in bodies which possess three unequal and rectangu- 
lar axes of elasticity. 
4th. The transformations of a series of plates perpendicular to any 
one of the three planes which pass through two opposite edges of the 
hexahedron are, in general, analogous to those of a series of plates cut 
round a line which divides into two equal parts the plane angle in- 
cluded between two of the three axes of elasticity in bodies where 
these axes are unequal and rectangular. 
5th. By means of the acoustic figures of a plate cut in a prism of 
rock crystal, nearly parallel to the axis, and not parallel to the two 
faces of the hexahedron, we can always distinguish which are the faces 
of the pyramid susceptible of cleavage. The same result may be 
obtained by the disposition of the modes of division of a plate taken 
nearly parallel to one of the faces of the pyramids. 
6th. Whatever be the direction of the plates, the optical axis, or its 
projection on their plane, always occupies a position on them which is in- 
timately connected with the arrangement of the acoustic lines: thus, 
for example, in all the plates cut round one of the edges of the base of 
the prism, the optical axis, or its projection, invariably corresponds with 
one of the two straight lines which compose the nodal system formed 
of two lines which intersect each other rectangularly. 
Although there is doubtless a great analogy between the phenomena 
which rock crystal has just presented to us, and those we have observed 
in bodies in which the elasticity is different according to three direc- 
tions perpendicular to each other, nevertheless we are forced to acknow- 
ledge that, with respect to the mode of experiment we have employed 
in these researches, rock crystal cannot be placed in the number of 
substances with three rectangular and unequal axes of elasticity, and — 
still less in the number of those all the parts of which are symmetri- — 
cally arranged round a single straight line. For the same phenomena ~ 
