the different Cryftalline Forms. 405 
logous to thofe of the forms themfelves, and we fhall have 
fucceflively : 
_ 20, For the oblique prifm having rhombufes for its bafes 
the expreffion reprefented figure 7 ; 
2a. For the right prifm with asiinists bafes that feen 
figure 8 ; 
22. For the right prifm having rhombufes for its bafes 
that of figure 9 ; 
23. For the right prifm with fquare bafes that of figure 10, 
24. And, in the laft place, for the cube that of figure 11. 
Here the bafe only is denoted by letters, becaufe what takes 
place in regard to this bafe may be applied to any one of the 
other faces. 
25. For all thefe different primitive forms a method of 
figures, analogous to that which I have adopted for the 
oblique-angled parallelopipedon of figure 1, may be followed ; 
but the letters of the fame name, figured in the fame man- 
ner, need not be repeated. 
One example will ferve to give an idea of this method, 
Let ar (fig. 12.) be the moft common variety of the cymo- 
phane, the nucleus of which is a rectangular parallelopipedon, 
Juch as that feen fig. 8. he fign of the fecondary cryftal , 
will be, MT?*G G? BA? 3A, in which M correfponds to 
gobinr, T to bets, °G to fgnm, G’ to betl, B to 
dacf, orbace, A te cf go, and EM toce bo. 
To illuftrate better the fteps which have conducted to this 
expreffion, let us point out, for a moment, all the angles 
and all the edges by as many particular letters, as if the pa- 
rallelopipedon were oblique-angled. (See fig. 13. ) . 
mm fign then will become MT 7G H? B F E? 20; but 
by comparing figure 13 with fi iguré 8, we fee that H = G, 
F = B, O = A; by fubftituting then in the place of the firtt 
letters their values, we fhall have MT *G G BB A? 
which amounts to the expreffion above fhewn, by fuppreffing ng 
the ufelefs repetition of B. 
90. It refults from the preceding, that we muft avoid con- 
Dd3 founding, 
