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 watei 

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

 nary temperature. When a right rhombic prisnfof sulphate 

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

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

 bunches of crystals shoot forth in the intej$or of the speci- 

 men ; and m 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 afme form may be ob- 

 tained by evaporating a solution at thif temperature or above* 

 it. Many other similar cases might J5e cited, but these serve 

 to explain the principle in view. 



IRREGULARITIES 



CRYSTALS. 



Before concluding this subj/ct, a few remarks may be 

 added on the irregularities of drystals. 



Crystals of the same form yary 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 



^W 



n 



*^^ggpt 



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

 he same form wjth some faces very much enlarged, and 



ethers 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 ofttV irregularities of crystals'? 



