MINERALOGY. 



451 



. Lateral edges are formed by the junction of two la- 

 teral planed, as in the prism and pyramid ; but in 

 the table, where the lateral planes do not meet, the 

 lateral edge* are those formed by the meeting of the 

 lateral planes and the terminal planes ; or we say, 

 they are the edges of the lateral faces of the table, so 

 that there are eight lateral edges in a four-sided 

 table, &c. Terminal edges are formed by the junc- 

 tion of lateral and terminal planes, as in the prism 

 and pyramid ; or they are those that surround the 

 terminal planes in the prism or the base of the pyra- 

 mid : they are also formed by the junction of two 

 terminal planes, as in the table *. 



5. Solid angle*. The point in which three or more 

 planes meet, is called a tolid angle. 



II. The Varieties or Kinds of the Fundamental Figure. 



The rie- Werner admits seven fundamental figures, vix. ico*- 

 t . or hear on, dodecahedron, hexahedron, prism, pyramid, table, 



ki.d*ofthe ndfc*. 



1. Icosahttlron is a solid having twenty equilateral tri- 

 angular planes, that meet together under nearly equal 

 ot>tue angles ; and of twelve solid angles, so that 



-e are always five planes to form an angle. Fig. 

 1. I'.ate (VfXCVl. It is rare. Example, Iron- 

 pyrite*. 



2. Dodecahedron has twelve regular pentagonal planes 

 that meet under equal obtuse angles ; and of twenty 

 solid angles. Fig. 2. It occurs but seldom. Ex- 

 ample, I run- pyrites. 



3. Hexahedron is a solid, having six square planes and 

 eight solid angles. It includes the cube, Fig. 3. 

 and the rhomboid, Fig. 4. which is sometimes con- 

 sidered as a double three-sided pyramid, in which 

 the Literal planes of the one are set on the lateral 

 edges of the other. It is very frequent. Example, 

 Calcareous- spar. 



4. Prism has an indeterminate number of quadrangular 

 lateral planes, terminated by two equal terminal 



tat tgure*. 



PlAT 

 CCCXCVI. 



Pig. 1. 



p,g. *. 



fig. S, 1. 



F.,. , 



Fig. 





parallel to each other, and having as many 

 side* as the prism has lateral planes. Fig. 5. 'I hi- 

 is the most frequent of the fundamental figures. Ex- 

 ample, Calcareous-spar. 



ramul has an indeterminate number of triangular 

 lateral planes converging to a point, and a base pos- 

 sessing as many sides as the figure has lateral planes. 

 Fig. 1 -'. The terminal point is called the summit 

 or apex, and the flat part the base. It occurs very 

 often. Example, Calcareous- spar. 

 6. Table has two equal and lateral planes, which are 

 very large in companion of the others, and which are 

 bounded by an indeterminate number of small four- 

 sided terminal planes. Fig. 15. It is but a very 

 short prum. It is proper to observe, that the parts 

 of the table are not denominated as those in the 

 prism, but inversely, the lateral planes of the table 

 corresponding to the terminal planes of the prism, 

 and the terminal planes of the table to the lateral 

 planes of the prism. It does not occur very often. 

 Example, Heavy-spar. 



Pif. It, to. 7. I*ens has two curved faces or planes, Figs. 1<). and 

 SO. It occurs but seldom. Example, Sparry-iron. 



III. The Differences of each Fundamental Figure in par- 

 ticular. 



Here we have to determine, 1. The Simplicity. S. The diffe- 

 Number of Planes. 3. Proportional size of the planes to rences of 

 one another. 4. Direction of the planes. 5. Angles n- ** ** 

 der nhick the planes meet. 6. Plenitude, or fulness nf the g e ^ r * in 



particular. 



1. Simplicity. 



With respect to simplicity, the fundamental figures Simplicity, 

 are either simple or double. This distinction, however, is 

 confined to the pyramid, as the other six kinds of pri- 

 mitive figures occur simple only. Fig. 13. is a simple Pig- 1*- 'S- 

 pyramid ; and Fig. 1 3. a double pyramid. 



' The simple figure is also distinguished, in regard of 

 its position, into erect or inverted, according as it ad- 

 heres by its bate or its summit. The inverted has 

 hitherto occurred only in calcareous spar, and is very 

 rare. 



In the double figure, we have to attend to the plac- 

 ing of the lateral planes ; thus, the lateral planes of the 

 one pyramid are placed either straight or oblique on the 

 lateral planes of the other pyramid. In Fig. 13. they 

 are placed straight; and in Fig. 14. they are placed ob- F 'g- !* 

 liquely ; or the lateral planes of the one pyramid are 

 set either on the lateral edges, as in Fig. 35. or on the Fig. IS. 

 lateral planes of the other, as in Fig. 13. 



2. Number of Plane*. 



The number of planes in the icos.ihedron, dodeothe- Number of 

 (Iron, hexahedron, and lens, is always determinate, but P lanei - 

 in the prism, pyramid, and table, is indeterminate. In 



the prism and pyramid, it is only the lateral planes that 

 vary in number, but in the table it is the terminal 

 plarles. 



The prism occurs with three, four, six, eight, 

 nine, and twelve lateral planes. The trihedral, or 

 three-sided, occurs in schorl and tourmaline. The 

 four-sided, or tetrahedral prism, Fig. 5. occurs very Fi fr * 

 often ; we have examples of it in felspar, xcolite, zir- 

 con, and heavy-spar. The six-skied, or hcxahedral 

 prism, Fig. 8. occurs very often, and is the most F * * 

 common prismatic crystallization; quartz, emerald, 

 beryl, calcareous spar, heavy spar, and actynolitc, 

 afford examples of it. The octahedral, or eight- 

 sided prism, is rare ; it occurs in augite and topaz. 

 The nine and twelve-sided prisms are merely varieties 

 of the preceding figures ; the first is formed by the 

 bevelling of the lateral edges of the trihedral prism, 

 the other by the truncation of the lateral edges of 

 the six-tided prism. Meryl affords an exitmple of the 

 twelve-sided, and tourmaline of the nine-side 1 prJMn. 



The pyramid occurs with three, four, six, and eight 

 sides. The three-sided pyramid, Fig. ;>. is either single Vlg ' 9- 

 or double ; of the single we have examples in grey 

 copper ore, spinel, copper pyrites, and many other mi- 

 nerals. Examples of the second occur in calcareous 

 spar, as in Fig. 10. The four-sided pyramid is the ' 

 roost common, and is always double, W, 11.; when B ' 

 it appears single, the one half is either hid in part, or 



The terminal cdgn in the Table are those that meaiurc iti Uiicknen. 



