GUIDE TO THE' MIXERALOGIC COLLECTIONS 



5 



called quartz; a white, pink or salmon colored mineral which 

 shows on the fracture a series of smooth surfaces and which is 

 called a feldspar; and a black fibrous mineral which is known 

 as hornblende. 



Crystallization 



When a substance in the condition of a liquid or a gas becomes 

 solid it is often seen that this solid has a regular outline, smooth, 

 bright sides or faces and sharp angles. This results from the 

 fact that the particles or molecules of the substance, which 

 while it was liquid or gaseous rolled about on one another, 

 have been in some way arranged, grouped and built up. To illus- 

 trate this, suppose a quantity of small shot to be poured into 

 a glass, the shot will represent the molecules of a substance in 



Fig. 1 Fig. 2 Fig. 3 



the liquid state, as for example a solution of alum. If, now, we 

 suppose these same shot to be coated with varnish or glue so that 

 they will adhere to each other and imagine them grouped as 

 shown in fig. 1 they will represent the arrangement of the mole- 

 cules of the alum after it has become solid or crystallized. This 

 arranging, grouping and piling up of the molecules is called 

 crystallization and the solid formed in this way is called a crys- 

 tal. Fig. 2 and 3 show the shot arranged to reproduce two com- 

 mon forms of crystals. 



There are many common examples of crystallization. The 

 snowflakes, which are formed by the cooling of watery vapor in 

 the air, are composed of small crystals which are quite apparent 

 to the eye and are often of great beauty and regularity of form. 

 The same may be said of the frost which forms on a window 



