152 SAVART ON THE ELASTICITY OF CRYSTALLIZED BODIES. 
sounds of the plates parallel to these planes become equal, and the two 
modes of division gradually transform themselves into each other, by 
turning round two fixed points, which, for this reason, I have called xo- 
dal centres. 
9th. The numbers of vibrations are only indirectly connected with 
the modes of division, since two similar nodal figures, as in No. 3, fig. 8, 
and in No. 3, fig. 14, are accompanied by very different sounds ; whilst, 
on the other side, the same sounds are produced on the occurrence of 
very different figures, as is the ease for No. 5 of fig. 8. 
10th. Lastly, a more general consequence which may be deduced 
from the different facts we have just examined is, that when a circular 
plate does not possess the same properties in every direction, or, in other 
words, when the parts of which it consists are not symmetrically arranged 
round its centre, the modes of division of which it is susceptible assume 
positions determined by the peculiar structure of the body; and that 
each mode of division, considered separately, may always, subject how- 
ever to alternations more or less considerable, establish themselves in 
two positions equally determined, so that it may be said that, in hetero- 
geneous circular plates, all the modes of division are double. 
By the aid of these data, which are no doubt still very few and im- 
perfect, a notion may be formed, to a certain point, of the elastic state 
of crystallized bodies, by submitting them to the same mode of investi- 
gation: this is what we have attempted for rock crystal, in a series of 
experiments which will be the subject of § 111. of this Memoir. 
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