the different Crpflalline Forms. 409 
the figures I fhall- not enlarge on: it, as it may be eafily de- 
duced from what we have adopted in regard to quadrangular 
prifms. But, it fometimes happens that three of the folid 
angles, taken alternately, are replaced by facets, while the in- 
termediary angles remain untouched. In that cafe the ex- 
preffion of the prifm will be that feen fig. 24. 
34. The rhomboidal dodecaedron, in certain fpecies, as that 
of red filver ore, has fix of its faces which perform the func-~ 
tions of the planes of a prifm; while the fix other faces en- 
ter into the analogy of rhomboids (29) ; fo that the faces of 
each order may undergo particular decrements, independently 
of thofe which regard the faces of the other order. Figure 
25 reprefents the expreffion of this dodecaedron. Each face 
of the fummit, in the fame cafe, may be confidered as the 
bafe of an obliqve quadrilateral prifm (20), and the adjacent 
planes as belonging to the fame prifm. Thus the manner 
of placing the letters which indicate the decrements, and 
the figures that accompany thefe letters, will be analogous 
to that which takes place in quadrilateral prifms. 
35. In other fpecies, fuch as that of the garnet and ful- 
phure of zinc, each folid angle, compofed of three planes, 
may be affimilated to the fummit of an obtufe rhomboid; 
‘and thus by employing figures only for one face we tal 
have the expreflion raprateried by figure 26, 
36. We fhall not employ the fign of the dodecaedron 
with ifofceles triangular planes, becaufe it is more natural to 
: fubftitute the rhomboid from which it arifes, as we have 
more fimple laws of decrement. 
* 37. It remains to give the means of reprefenting a particular 
‘cafe which takes place in certain cryftals, where the parts 
‘oppofite to thofe that obey certain laws of decrement remain 
untouched, or are modified by different laws.” ‘This cafe 
belongs, in a particular manner, to turmalins; and. it vis 
aly then to indicate the difference by means of\a zero. 
For example, in the very obtufe turmalin, the nucleus ‘of 
‘which we ‘hall fuppofe  reprefented) figure-\15;, the »prifm 
ee is enneagonal has fix of its planes produced by fub- 
tractions 
