IRREGULARITIES OF CRYSTALS. 



45 



The forms of a dimorphous substance differ in stability 

 Aragonite when heated gently falls to powder, arising from 

 a change in the condition of its particles. Aragonite has 

 been obtained by evaporating a solution of lime over a water 

 bath, and calc spar when the same was evaporated at the ordi- 

 nary temperature. When a right rhombic prism of sulphate 

 of zinc (which is dimorphous) is heated to 126° F. certain 

 points in its surface become opaque, and from these points, 

 bunches of crystals shoot forth in the interior of the speci- 

 men ; and in a short time the whole is converted into an 

 aggregate of these crystals, diverging from several centers on 

 the surface of the original crystal. These small crystals 

 are oblique rhombic prisms ; and the same form may be ob- 

 tained by evaporating a solution at this temperature or above 

 it. Many other similar cases might be cited, but these serve 

 to explain the principle in view. 



IRREGULARITIES OF CRYSTALS. 



Before concluding this subject, a few remarks may be 

 added on the irregularities of crystals. 



Crystals of the same form vary much in length, and in the 

 size of corresponding faces. The same mineral may occur 

 in very short prisms, or in long and slender prisms : and 

 some planes may be so enlarged as to obliterate others ; 

 a few figures of quartz crystals will illustrate* these pecu- 

 liarities. 



79 80 81 82 83 



fa 



Figure 79 is the regular form of the crystal. Figure 80 is 



the same form with some faces very much enlarged, and 



others very small. Figure 81 is a very short prism and 



yramid of quartz, such as is often seen attached to the 



urface of rocks ; and figure 82 is a similar form very much 



longated. Notwithstanding all these variations, every angle 



What are some ofth** irregularities of crystals? 



