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CKTSTALLOGK APHT. 



square prism, or else of a rhombic prism ; and if they are at 

 right angles to one another that they are those of the former. 



This subject is further illustrated beyond. 



In the following pagea (1) the Systems of Crystallization and 

 the Forms and Structure of Crystals are first considered ; next, 

 (2) Compound, or Twin Crystals; and then (3) Crystalline 

 Aggregates. 



1. SYSTEMS OF CRYSTALLIZATION : FORMS 

 AND STRUCTURE OF CRYSTALS. 



The forms of crystals are exceedingly various, while the sys- 

 tems of crystallization, based on their mathematical distinctions, 

 are only six in number. Some of the simplest of the forms 

 under these six systems are the prisms represented in the fol- 

 lowing figures; and by a study of these forms the distinctions 





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of the six systems will become apparent. These prisms are all 

 four-sided, excepting the last, which is six-sided. In them the 

 planes of the top and bottom, and any planes that might be 

 made parallel to these, are called the basal planes, and the sides 

 the lateral planes. An imaginary line joining the centres of 

 the bases (c in figures 1 to 8) is called the vertical axis, and the 



