CRYSTALS UNDER THE MICROSCOPE 169 



crystal form. Such replacement forms are met with in minerals 

 and are known as pseudomorphs. 



When a crystalline substance is found with its own crystal 

 outlines it is said to be idiomorphic. 



When a crystal has opposite ends different, due to dissimilar 

 faces, it is termed hemimorphic. 



Two crystals may unite to form a double or twin crystal. 

 Unions in threes or fours are less frequent. 



Many chemical compounds are known, however, which form 

 more than one kind of crystal. Such substances are said to be 

 dimorphous, trimorphous or polymorphous, according to the num- 

 ber of observed kinds of crystals which they form. 



The six crystal systems are as follows, each comprising the 

 number of simple generic forms indicated : 



CTC< Number of Simple 



S y stem Generic Forms 



1. Isometric Seven. 



2. Tetragonal Seven. 



3. Hexagonal Seven. 



4. Orthorhombic Four 



5. Monoclinic Four. 



6. Triclinic Four. 



A crystal is said to be holohedral when all its planes are present. 

 When one-half the planes are present (in accordance with an 

 established law) the crystal is hemihedral; and if only one- 

 quarter the possible planes, the crystal is called tetartohedral. 



Crystal aggregates uniting in such a manner as to yield branch- 

 ing, fern-like, moss-like or tree-like forms are called dendrites, 

 and the mass is termed a dendritic mass. If the aggregate con- 

 sists of more or less long hair-like twisted, curved or bent crys- 

 tals, it is said to have a trichiten structure, and the individual 

 hair-like bodies are called trichites. But when the tiny long 

 narrow crystals are straight and resemble needles, the crystals 

 are said to be acicular. Tiny globular masses of radiating, 

 acicular crystals are called spherulites or sphero-crystals. When 

 these radiating aggregates consist of anisotropic crystals they 



