14 CRYSTALLOGRAPHY. 
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 i is further illustrated beyond. 
In the following pages (1) the Systems of Crystallization and 
the Forms and Structure of Crystals are first considered; next, 
(2) Compound, or Twin Cr ystals ; ; and then (3) Crystalline 
A geregates. 
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 st 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 

of the six systems will become apparent. These prisms are ali 
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 dasal planes, and the sides 
the /ateral planes. An imaginary line joining the centres of 
the bases (c in figures 1 to 8) is called the vertical axis, and the 
