TSIMETBIO, OB OKTHORHOMBIC SYSTEM. 27 



in two directions at right angles with one another. The 

 lateral planes of either square prism are alike in lustre and 

 markings. 



III. TRIMETRIC, OR ORTHORHOMBIC SYSTEM. 



1. Descriptions of Forms. — The crystals under the trimctrio 

 system vary from rectangular to rhombic prisms and rhombic 

 octahedrons, and include various combinations of such forms. 

 Pigs. 1 to 7 are a few of those of the species barite, and figs. 8 

 to 10 of crystals of sulphur. 



4. czzzz^^ ® 



BARITE. 



SULPHUR. 



Fig. 11 represents a rectangular prism (diametral prism), 

 and fig. 12 a rhombic prism, each with the axes. The axes 

 connect the centres of the opposite planes in the former ; but 

 in the latter the lateral axes connect the centres of the oppo-. 

 site edges. Of the two lateral axes the longer is called the 

 macrodiagonal, and the shorter the br achy diagonal. The verti- 

 cal section containing the former is the macrodiagonal section 

 and that containing the latter, the br achy diagonal section. 



In the rectangular prism, fig. 11, only opposite planes are alike, 

 because the three axes are unequal. Of these planes, that oppo- 

 site to the larger lateral axis is called the macropinacoid, and 

 that opposite the shorter the brachypinacoid (from the Greek for 

 long and short, and a word signifying board or table). Each 



