DEFINITIONS. 5 



Fig. 9. 



An oblique angled parallelogram,* fig. 9, has its 

 opposite sides parallel, but its adjacent sides , b, and 

 its adjacent angles, c, d, unequal. 



Where certain forms of crystals are described with 

 reference to the rhomb as the figure of some of their 

 planes, they are termed rhombic.^" 



A parallelopiped is any solid contained within three 

 pairs of parallel planes. 



Crystals are conceived to be formed by the aggre- 

 gation of homogeneous molecules., which may be again 

 separated from each other mechanically, that is, by 

 splitting or otherwise breaking the crystal. 



These molecules, which relate properly to the crys- 

 tal, must be carefully distinguished from the elemen- 

 tary particles of which the mineral itself is composed. 



Sulphur and lead are the elementary particles, 

 which, by their chemical union, constitute galena; 

 but the molecules of galena are portions of the com- 

 pound crystalline mass, and are therefore to be re- 

 garded as homogeneous, in reference to the mass itself. 



* A parallelogram is any right lined quadrilateral plane figure, whose 

 ofifioiite sides are equal and parallel. 



f What is here called rhombic, most writers on this subject have, in 

 imitation of the French idiom, denominated rhomboidal; but as the term 

 rhomboid has been used in works on geometry to signify an oblique 

 angled parallelogram, and as the same term has also been already 

 appropriated in crystallography to a solid contained within six equal 

 rhombic planes, the application of the term rhomboidal to any other solid 

 seems to involve a degree of ambiguity. The term rhombic is, besides, 

 more conformable to the practice of our own language. 



