the different Cryftalline Forms. 407 
upon one and the fame vertical axis, has properly no, bafe, 
but only two, fummits, which are the extremities of the axis. 
Thefe angles and edges may be denoted as feen. figure 15. 
.The letter e makes known that the angle which bears it is 
fimilar to that marked with the fame large letter; fo that if 
all the lateral angles had their indications expreffed, the 
three neareft the upper fummit would bear the letter E, and 
the three next to the lower fummit, and which are vifibly 
oppofite to the firft, would have for their indicative letter ¢, 
As the rhomboid has its fix faces equal and fimilar, it is 
only neceffary to confider the decrements in regard to one of 
the faces, fuch as that which bears the letter P, becaufe all 
the reft are only repetitions of the latter. This being laid 
down, rft, the decrements which depart from the upper 
angle A, or from the upper edge B, will have their indicating 
figure placed below the letter A or B; 2d, thofe which de- 
part from the lateral angles E will be denoted by the fame 
letter written twice, one on the right, and the other on the 
left; 3d, in regard to thofe which depart from the lower 
angle ¢, or from the lower edge D, the figure deftined,to ex- 
Naa them will be placed above the letter e or D. 
Let us fuppofe, for example, that figure 16 reprefents ana- 
logical carbonat of lime, in which the vertical faces ecp g, 
ogrz, &c. refult from a decrement by two ranges on the 
angles e (fig. 15.); the oblique faces mdce, be go, &c. from 
a decrement by two ranges on the edges D, and the terminat- 
ing faces imeh, if th, ves a decrement by a range on the 
edges B, we fhall have the following fign ¢ D B. 
30. The other primitive forms, after what I ‘have faid in 
regard to the parallelopipedon, will be attended with no dif= 
ficulty. I fhall run them over in fucceffion. Figure 17 re- 
prefents the expreffion of the oGaedron with f{ealene trian+ 
gles ; figure 18 that of the oétacdron with ifofceles triangles, 
and figure rg that of the regular oftaedron. To, place the 
figures, which accompany the letters, we fhall conform ta 
sphat has been faid in regard to the rhomboid. Thus ( Je 18,) 
Dd4 wo 
