CRYSTALLOGRAPHY. Id 



In the study of this science, the learner will be introduced 

 first to the structure of minerals. The subject is treated of 

 under its usual name, crystallography 



CHAPTER II. 



crystallography: or the structure of minerals. 



Crystals : Crystallization. The regular forms which 

 minerals assume are called crystals, and the process by 

 which their formation takes place, is termed crystallization. 



Crystallization is the same as solidification. Whenever 

 a liquid becomes solid there is actual crystallization. Under 

 favorable circumstances regular crystals may form ; but 

 very commonly the solid is a mass of crystalline grains, as 

 is the case in statuary marble, or a loaf of white sugar. In 

 the case of the marble, crystallization commenced at myri- 

 ads of points at the same instant, and there was no room for 

 any to expand to a large size and regular outline. When 

 on the contrary, the process is slow, simple crystals often 

 inorease to a large size. 



We may understand this subject of crystallization by 

 watching a solution of salt, as it evaporates over a fire. Af- 

 ter a while, if the process is not too rapid, minute points of 

 salt appear at the surface, and these continue enlarging. 

 They are minute cubes when they begin, and they increase 

 regularly by additions to their sides, till finally they become 

 so heavy as to sink. In other cases, if the brine is boiled 

 away too rapidly, a mass of salt may be formed at the bot- 

 tom of the vessel, in which no regular crystals (cubes) can 

 be seen. Yet it is obvious that the same power of crystal- 

 lization was at work, and failed of yielding symmetrical 

 solids, because of the rapidity of the evaporation. Crystals 

 of salt have been found in the beds of this mineral a foot 

 or more in breadth, which had been formed by natural evapo- 

 ration ; and the whole bed is in all cases crystalline in the 

 structure of the salt. However finely the salt may be ground 



Explain the terms crystal and crystallization. Are solidification and 

 ■crystallization the same process ? Explain the different results of crys- 

 tallization by the example of salt. Is every grain, however minute 

 crystalline 1 



