408 Short Method of exprefing 
we mutt put the figure below for the decrements which pros 
ceed from A to B; above for thofe that depart from ae 
and at the fide for thofe which depart from FE. | 
If we wifhed to denote the refult of a’decrement by one 
range on all the angles of the regular ottaédron (fig. 19.), 
we fhould write A "A™; and to indicate the refult ofa de- 
crement by a range on all the edges we fhould write B B. 
The firft of thefe decrements produces a cube, and the fe- 
cond a dodecaedron with rhombules for its planes. . 
31. In fome kinds of cryftals, as thofe of the nitrat of 
potafh, the oftaedron, the furface of which is compofed of 
eight ifofceles triangles, fimilar four to four, muft be fituated 
as reprefented figure 20, in order that the fecondary cryftals 
may be in the moft natural pofition ;_ that is to fay, that the 
edges at the junction of the two pyramids compofing the 
octaedron may be, one in a vertical direction as F, and the 
reft in a horizontal dire€tion as B. By comparing fig. 20 with 
fig. 21 , where the Jetters have been placed as if all the angles 
and all the edges had peculiar functions, we fhall readily con- 
ceive the diftribution adopted figure 20, and brought back 
to the fymmetry of the real primitive figure ; for, in the pre- 
fent cafe, we have = A, D =C,G=J. The indicating 
figure muft be placed below the letter for decrements de- 
parting from B; and on one fide or below it for thofe de- 
parting from A, according as ae effeet {hall be dire&ted 
towards Bor F.. 
32. The tetraedron being ‘always regular, when. it fe 
comes the primitive form, its expreflion will be reprefented 
fi igure Liane) indicate, for example, a decrement by three 
ranges ‘on all the edges we mutt put B and B; ; and to denote 
one by two ranges, on all the angles, we mutt put A 7A%,.as 
in the cafe of the regular octaedron. site 
33. A fhort view of figure 23 will be fufficient to give an 
idea of the method of denoting the regular hexaedral prifin 
in ordinary cafes; and with regard to the manner of placing 
the 
