20 



MINERALOGY. 



called its cleavage planes. Minerals sometimes cleave 

 in only one direction, as common mica and foliated gyp- 

 sum, and sometimes in two, three, or four directions. 

 In many minerals it is difficult to effect cleavage ; in 

 some, as quartz, it is impossible, though even in this case 

 it has been done by heating the crystal, and then plung- 

 ing it into cold water. 



25. Primary Forms. While there is vast variety in 

 the forms of crystals as found in nature, mineralogists 

 have discovered by cleavage that the primary or funda- 

 mental forms are few in number. There are only thir- 

 teen of these forms, which are the solids that have been 

 obtained from the various minerals by cleavage. These 

 are divided into six classes, each class containing those 

 which can be produced by cleavage from each other. 

 The first contains the three solids represented in Figs. 9, 



Fig. 9. Fig. 10. Fig. 11. 



10, and 11, the cube having four equal square sides, the 

 octahedron having for sides eight equilateral triangles, 

 and the rhombic dodecahedron, whose twelve sides are 

 equal rhombic planes.* I will show by figures how the 



* The name dodecahedron is from the Greek words dodeka, twelve, 

 and hedra, base. A rhombic plane, or rhomb, is a plane with four 

 equal sides, those which are opposite being parallel, and the angles 

 being unequal, two of them being acute and two obtuse. It differs 

 from a square in being oblique-angled instead of right-angled. You 



see the difference in the two fig- 

 ures. The angles of the square 

 are all equal, but in the rhomb a 

 and its opposite angle are equal, 

 being obtuse, and b and its oppo- 

 site angle are equal, being acute. 



