176 



THE POPULAR EDUCATOR. 



in the four principal figures of the system (Figs. 15, 16, 17, 

 and 18). The two prisms and the two octahedra are formed in 

 the usual manner. In one case the axia terminates in the centre 

 of the faces, in the other in the angles. The way in which these 

 octahedra and right-square prisms blend with each other is shown 

 in Figs. 19 and 20. 

 Tinstone, ferrocyanide 

 of potassium, cyanide 

 of mercury, rutile, ana- 

 tase, and idocrase crys- 

 tallise in thia system. 



3. The Trimetric, 

 Eight Rectangular, or 

 Prismatic System. 

 This system has three 

 axes, all right angles, 

 but all unequal. The 

 effect of the lateral 

 axes being unequal 

 makes the bases of the 

 prism and octahedron 

 rectangles but not 

 squares (Figs. 21 and 

 22), and if the axes 

 terminate in the angles, 

 then the bases will be 

 rhombs, that is, paral- 

 lelograms with equal 

 aidea. The accompany- 

 ing figurea will indicate 

 this. Nitre, aragonite, 

 topaz, sulphate of 

 baryta, sulphur, and 

 stilbite belong to this 

 system. 



4. Tlie Monodinic or 

 Oblique System. The 

 axes of this system are 

 unequal in length, like 

 the last, but two of 

 them cut each other 

 not at right angles. 

 The effect of this is, 

 that the base of the 

 prism or octahedron ia 

 a parallelogram, but 

 not having equal aides. 

 Figs. 23, 24, and 25 

 will give the idea of 

 this system. Green 

 vitriol, sulphate of 

 soda, phosphate of soda, 

 sulphur crystallised 

 from its melted state, 

 and borax crystallises 

 in this system. 



5. The Triclinic, 

 Doubly Obligue, or 

 Anorthic System has 

 also three unequal axes, 

 but none of them inter- 

 sect at right angles. 

 The octahedron and 

 the prism of the system 

 are Figs. 26 and 27. 

 But few minerals ap- 

 pear in this system , 

 the most common are 

 blue vitriol (sulphate 



of copper), labradorite, anorthite, and axinite. 



6. The Hexagonal or Rhombohedral System has four axes, 

 three of them in the same plane and intersecting at angles of 

 60", and all equal; the fourth perpendicular to these, and vary- 

 ing in length. By joining the extremities of the three axes a 

 hexagon is formed, which is the base of the prism (therefore 

 six-sided) and of the hexagonal dodecahedron. These primary 

 forms are shown in Figs. 28 and 29. They appear in snow 

 crystals, beryl, tourmaline, and nitrate of soda; and the very 



common crystals of quartz are generally six-sided prisms ter- 

 minated by six-sided pyramids, aa in Fig. 30. Thia system ia 

 also called the rhombohedral, from the fact that the rhomb, 

 BO admirably ahown in calc-spar, is the hemihedral form of the 

 hexagonal dodecahedron ; that is, if the alternate faces of 



Fig. 28 be produced, 

 there being six of them, 

 they will form a six- 

 sided solid, whose re- 

 lation to the prism ia 

 best described by Fig. 

 31. It will be seen 

 that the edges of the 

 rhomb intersect the 

 faces of the prism in 

 six slanting lines. If 

 the points where these 

 lines meet the edges of 

 the prism be joined to 

 the extremities of the 

 longer axis, then the 

 scalenohedron will be 

 formed, a crystal which 

 ia found in a form of 

 carbonate of lime, 

 called dog-tooth spar. 



This glance at Crys- 

 tallography will be 

 sufficient to enable the 

 student to comprehend 

 the allusiona to the 

 subject in the following 

 lessons in Mineralogy. 

 It may be useful to 

 give the reader a brief 

 explanation of some of 

 the terms that have 

 been nsed in this and 

 the preceding lesson 

 in the names of the 

 various systems under 

 which crystals are clas- 

 sified. 



Monometric means 

 having one measure- 

 ment or equal measure- 

 ment, from the Greek 

 fj.ovos (mon'-os), one, 

 and (nfTpeca (met'-re-o), 

 I measure; the mono- 

 metric system being 

 distinguished by equa- 

 lity of axes. Dimetric 

 means having two mea. 

 surements, from the 

 Greek Sis (dis), twice, 

 etc.; crystals in this 

 system having one 

 longer axis and two 

 shorter ones, which 

 are of the same length. 

 Trimetric means hav- 

 ing three measure- 

 ments, from the Greek 

 rptis (trice), three, 

 etc. ; the crystals in 

 this system having 

 three axes, all of 

 which differ in length. 



MonocUnic means having one sloping axis, from IJLOVOS , one, and 

 K\iv<e (kli'-no), I incline or bend ; crystals of this system having 

 one axis which is not rectangular to the other two. The mean- 

 ing of Triclinic is having three axes at oblique angles to one 

 another. Its derivation is obvious. Hexagonal means six-sided 

 or six-angled, from the Greek e (hex), six, and ycavia (go'-ni-a), 

 an angle ; and Rhombohedral, having its sides in the form of a 

 rhombus, from pofj.&os (rhom'-bos), a figure whose four sides are 

 equal, but its angles are not right angles, and e5pa (ed-ra), a side. 



