18 MINERALOGY. 



substance as it can take up, and then allowing the solu- 

 tion to cool. The formation of frost is really crystalliza- 

 tion by deposition from a solution, for the water, depos- 

 ited in -solid form was dissolved in the air ( 96, Part 

 II.). Warm air will hold more water in solution than 

 cold, and, therefore, when warm air becomes cooled, it 

 deposits some of its water in the shape of dew when the 

 cold is moderate, and in the solid form when it is severe. 

 2. A liquid or a gas may be converted into a solid with 

 a crystalline arrangement. Various examples of this 

 are familiar to you, as the formation of ice from water, 

 the freezing of mercury, and the solidification of melted 

 metals on cooling. The most common example which 

 we have of the conversion of a gas or vapor into a crys- 

 talline solid is in the formation of frost from the vapor 

 of water in the air. 3. A mineral substance which is 

 not crystallized may become so by heat, and perhaps 

 some other agencies acting in connection with this. 

 Many rocks, as you will see illustrated in another part 

 of this book, owe their crystalline character to this cause. 

 It is in this way that marble has been made out of com- 

 mon chalk or limestone, it differing from them not in 

 chemical composition, but merely in being crystalline. 



20. Water of Crystallization. The crystals of many 

 minerals have water incorporated with them, and, as its 

 presence is essential to their crystalline condition, it is 

 termed their water of crystallization. For a more full 

 statement in regard to this, I refer you to 164, Part II. 



21. Amorphous and Dimorphous Minerals. A mineral 

 is said to be amorphous when it is destitute of all trace 

 of crystalline form. The term comes from two Greek 

 words, a, without, and morphe, form. When a mineral 

 appears in crystals of two forms, sometimes the one and 

 sometimes the other, it is said to be dimorphous, the first 

 part of the term coming from the Greek word dis, twice. 



22. Arrangements of Crystals. Crystals forming in 

 groups are arranged variously by the influence of cir- 



