PHYSICAL PROPERTIES 



265 



Where samples for analysis are required it is advisable to pass 

 the material through the separator several times, care being taken 



FIG. 376. The Wesphal Balance. 



to have the specific gravity of the heavy liquid nicely adjusted to 

 the specific gravities of the minerals to be separated. 



Terms used in the description of mineral aggregates. Indi- 

 vidual crystals, even though of the same species and combinations 

 of the same forms, may differ greatly in appearance. The general 

 shape of a crystal will depend upon the crystal form 

 which predominates in the combination; thus various 

 habits arise which are described by the following 

 terms : 



Tabular is when a pair of parallel faces, as the base 

 or pinacoid, predominate. The crystal is short in one 

 'linrtion and extended in the other two, as in some 

 barites, tabular parallel to the base, Fig. 378. 



Prismatic is when the individuals are elongated in the 

 direction of any one axis, usually the vertical axis, as in 

 quartz. 



Short and stout prismatic crystals are said to be of 

 columnar habit, as beryl. Fio. 377. 



