MINERALOGY. 



to the Wernerian metliod, is a four-fided prifm, acuminated 

 at each extremity by four planes fet on the lateral planes. 

 Fig. 23. is a four-fided prifm acuminated by four planes 

 fet on the lateral edges. In the fecond inftance, the acumi- 

 nating planes are rhomboidal ; in the firft inftance, triangular. 

 Fig. 24. is the equiangular fix-fided prifm, acuminated at 

 each extremity by iix planes fet on the lateral planes. 

 Fig. 25. is a fimilar prifm acuminated by three planes at 

 each extremity fet on the alternate lateral planes : and 

 Jg. 26. is alfo a fimilar fix-fided prifm acuminated by three 

 planes at each extremity, fet on the alternate lateral edges. 

 The planes at the oppofite extremities in Jgs. 25 and 26. 

 are fet in different planes or edges, and are what the Ger- 

 mans call unconformable. 



The oftahedron, or double four-fided pyramid {fg. 27.), 

 IS a common form of cryftals : it is frequently truncated or 

 bevelled. Fig. 28. reprefents the oftahedron bevelled at 

 each of the angles. Fig. 29. is a double eight -fided pyra- 

 mid acuminated by four planes at each extremity, fet on the 

 alternate lateral edges of the pyramid, thus forming a cryf- 

 tal with twenty-four faces, a form common in the leucite. 

 The table may be bevelled on the furrounding edges, as in 

 Jig. 30 ; but in this figure, the Wernerian mineralogifts call 

 the furrounding fides the terminal planes, and the larger 

 fides the lateral planes, a deviation for which there does not 

 appeal- any fufficient reafon, and which is liable to intro- 

 duce confufion into the defcription of cryftals. Some of 

 the forms here referred to, particidarly figs. 22 and 34, may 

 be more fimply defcribed as four-fided and fix-fided prifms, 

 terminated at each extremity by four-fided or fix-fided pyra- 

 mids. The edges of a cryftal may fometimes be doubly 

 bevelled, or may be bevelled, and the edge of the bevelment 

 truncated. Such modifications are better defcribed as re- 

 placements of the edges by three, four, or more fmall 

 planes or fecondary faces. A very long prifm is called 

 a capillary cryftal, if the diameter be too fmall to render 

 the faces diftinft. A very Ihort prifm, in which the length 

 is very fmall in proportion to the thicknefs, may be regarded 

 as a tabular cryftal. Moft cryftals may be very intelligibly 

 defcribed by alTuming the fundamental forms of Werner 

 varioufly modified. We are not, however, to fuppofe, that 

 Werner himfelf intended to convey the idea that nature firft 

 made cryftals complete, and then cut away the angles and 

 edges ; he only exprefles, by the terms truncation and 

 bevelment, the appearance the cryftal prefents to the eye. 

 The primitive forms, or the true fundamental forms of 

 cryftals, as given by Haiiy, are enumerated under the article 

 Crystal; hut Plate \ II. Cryjlallization, comprifes thofe 

 forms, and alfo thofe of the integrant molecule. The pri- 

 mitive forms are, the parallelepiped, including the cube 

 and rhomb {fgs. i and 2.), and the reftangular -table [Jigs. 7 

 and 8. ), the oftahedron [Jig. 27. ), the tetrahedron {Jig. 13.), 

 the hexagonal prifm [fg.s-), the rhomboidal dodecahedron 

 (JS- 1 2-), and the dodecahedron with triangular faces {Jg. 

 14. ) The integrant molecules are, the tetrahedron {Jg. 13.), 

 the three-fided prifm, and the cube {Jg. 1 . ) 



In nature, we rarely find cryftals entirely perfed and fym- 

 metrical. If, fays Mr. Aikin, the ftudent ftiould imagine 

 that the real cryftals of minerals, fuch as nature prefents 

 them, are formed with the precifion that charafterifes the 

 models of the cryftallographer, he will in general find him- 

 felf much miftaken. By far the greater number of cryftals 

 are either imbedded in other fubftances, from which it is 

 difficult to difengage them without much injury, or inhere 

 by one extremity in amorphous or uncryftallized matter of 

 the fame nature wit.- themfelves. Hence it is, that few 

 prifms occur both terminations of which are entire. Not 



unfrequently alfo cryftals, by being formed in nariow clefts, 

 are comprefled, or in other ways varioufly mutilated, and 

 thus perplex even the moft fl'iilful mineralogitt. Their 

 minutenefs too, when the parts are much complicated, is 

 frequently fuch as to elude the keeneft eye and the moft 

 adroit ufe of the goniometer. 



It is often by no means eafy to diftinguifli genuine from 

 fpurious cryftals. The latter are generally fuppofed to have 

 been formed in cavities occafioned by the decompofition 

 of real cryftals. Thefe are called cafts. Spurious or fup- 

 pofititious cryftals are either cafts or ineruftations ; the lat- 

 ter occurs when a mineral is depofited over a pre-exifting 

 cr^'ftal and afliimes its figure. The cryftal either remains 

 forming a nucleus, or it difappears, and the fuppofititious 

 cryftal is hollow. 



In the Wernerian oryftognofy, the magnitude of cryf- 

 tals and their mode of aggregation are minutely enumerated. 

 (See Oryctog.vosy. ) In the laft edition of Mr. Jamefon's 

 external charafters, he defines the fcopiform or fafcicular 

 aggregation as " compofed of a number of thin prifmatic 

 cryftals, diverging from their point of attachment, and form- 

 ing a kind of fafciculus or bundle; example, zeolite." The 

 manipular, or flieaf-like, confifts of a number of cryftals that 

 diverge towards both ends and are narrower in the middle, 

 thus refembling a ITieaf ; examples, zeolite and prehnite. 



Scalar-wife is when -many cubical cryftals are arranged 

 like the fteps of a ftair ; example, cubes of corneous filver- 

 ore. 



The furface of cryftals is either fmooth or ftreaked, the 

 ftreaks fometimes crofs longitudinally and fometimes tranf- 

 verfely. Thefe ftreaks are deferving of notice, as they 

 frequently ferve to indicate the ftrufture of cryftals. See 

 Crystallogkaphy, Addenda. 



Eledricity is a charafter peculiar to a fmall number of 

 cryftallized minerals, that exhibit the pofitive and negative 

 eledricity at their extremities on being heated. This pro- 

 perty was firft difcovered in the tourmaline. (See Tour- 

 maline.) The points which exhibit electricity are called 

 the eleftric poles. In order to diftinguilli theie poles from 

 each other, the following fimple apparatus is employed. 

 (See Crystallography, PlateYV.Jg.<)2. A.) It con- 

 fifts of a needle of copper or filver, having at each end two 

 fmall balls a, b ; this needle, like the common compafs- 

 needle, is moveable upon a pivot, having a very fine point, 

 and at the bottom a ftand or foot. The needle and ftem 

 are infulated by placing the foot on a plate of wax or refin. 

 To ufe the apparatus, we place one finger on the ftem, and 

 prefent near to the needle a ftick of feahng-wax, made elec- 

 trical by rubbing, then withdraw the finger, and afterwards 

 the ftick of fealing-wax, and the needle will be pofitively 

 elecflrified ; and when a cryftal eleftrified by heat, and held 

 by a pair of fmall pincers in an infulated handle, as at B, 

 is brought fo that the pofitive or negative pole may ap- 

 proach one of the balls a, a, b, it will be attradled or 

 repelled. The eleftricity of the needle will be perceived 

 a quarter of an hour, or longer, and may be rendered more 

 or lefs ftrong by varying the diftance at which the ftick 

 of fealing-wax is held. It may be proper to obferve that 

 many minerals become eleftric by friftion. 



Magnetifm is a charafter which occurs principally in 

 ores of iron, or in minerals that contain a portion of iron, or 

 iron in a ftate of black oxyd. 



The above are the moft important phyfical charafters, 

 compriilng thofe which are called the external characters 

 by Werner, and thofe to which he reftridls the term phy- 

 fical. 



Specific Gravity, or weight, is one of the moft im- 

 portant 



