MINERALOGY. 391 



species, wherever it is found crystallized, is limited to certain simple 

 forms, or groups of forms, by which its composition and properties 

 may often be recognized. Minerals which have no regular shape, 

 are said to be amorphous. Others have what is called imitative 

 forms ; as dentiform, or tooth-shaped ; filiform, or thread-like ; 

 dendriform, or tree-shaped ; pectiform, or comb-shaped ; reniform, 

 or kidney- shaped ; and botryoidal, resembling a cluster of grapes. 

 But of all the mineral species known, the larger portion are found, 

 at least occasionally, having a regular crystalline form, subject to 

 mathematical laws : and these forms alone are the object of the pre- 

 sent section. 



A crystal, is an inorganic solid, bounded by plane surfaces, and 

 usually possessing a homogeneous structure. The bounding surfaces 

 are called faces ; the lines of intersection, edges ; the angles of the 

 faces themselves, plane angles; those which they form with each 

 other, interfacial angles ; and an angle formed by three or more 

 faces, meeting in a point, is called a solid angle. The forms of crys- 

 tals are considered as either primary, or secondary ; the latter being 

 derivable from the former, either by excision or by accretion. Crystals 

 generally admit of cleavage, or splitting ; and when this can take 

 place only in one direction, the cleavage is said to be single ; but 

 when in two directions, double ; and it may also be triple or quadru- 

 ple. When the edges or angles of a crystal are cut off by a plane 

 equally inclined to the sides which it cuts, they are said to be trun- 

 cated ; but when the edge is replaced by two faces, equally inclined; 

 or an angle replaced by as many such planes as there are contiguous 

 faces, they are then said to be bevelled. 



All those crystalline forms which are reducible to one and the same 

 primary form, are said to constitute a crystalline system. There are 

 seven of these systems, distinguished by the relations of their axes. 

 In the monometric, or regular system, the three axes are equal, and 

 at right angles to each other ; as in the cube, the axes of which are 

 the lines joining the centres of the opposite sides; and in the regular 

 octahedron, and the rhombic dodecahedron, derived from the cube 

 by truncating all or a part of the edges ; these figures having their 

 axes joining the vertices of opposite angles. The dimetric system, 

 has one axis either longer or shorter than the other two, as in the right 

 square prism : and the trimetric system has all the axes unequal, 

 but still at right angles ; as in the right rectangular prism. The 

 other systems, which we have no room here to describe, have one or 

 more of their axes oblique. An instrument for measuring the angles 

 of a crystal, is called a goniometer ; one form of which, invented by 

 Wollaston, employs for this purpose the reflection of light. 



2. Under the head of Idiographic Mineralogy, we would treat 

 of the various physical and chemical properties of minerals ; which 

 serve, often in part, and for amorphous minerals entirely, to aid us 

 in recognizing them. We speak here only of simple minerals ; all 

 the separable particles of which are homogeneous : as the study of 

 compound minerals formed by the aggregation of two or more simple 

 ones, belongs to the study of Geology. The physical or external 

 properties of minerals, are those which are obvious by mere inspec- 



