ie) 4 
SYSTEMS OF CRYSTALLIZATION. 1 
diagonals @ and 6, drawn in a plane parallel to the base, are the 
lateral ases. 
Fig. 1 represents a cube. It has all its planes square (like 
fig. 9), and all its plane and solid angles, right angles, and the 
three axes consequently cross at right angles (or, in other 
9. 10. 11 
oe 
words, make rectangular intersections) and are equal. Itis an 
example under the first of the systems of crystallization, which 
system, in allusion to the equality of the axes, is called the 
Lsometric system, from the Greek for equal and measure. 
Fig. 2 represents an erect or right square prisin having all its 
plane angles and solid angles rectangular. The base is square 
or a tetragon, and consequently the lateral awes are equal and 
rectangular im their intersections ; but, unlike a cube, the verte- 
cal axis 1s unequal to the lateral, There are hence, in the square 
prism, axes of two kinds making rectangular intersections. The 
system is hence called, in allusion to the two kinds of axes, the 
Dimetric system, or, in allusion to the tetragonal base, the 7¢- 
tragonal system. 
Fig. 3 represents an erect or right rectangular prism, in 
which, also, the plane angles and solid angles are rectangular. 
The base is a rectangle (fig. 10), and consequently the lateral 
axes, connecting the centres of the opposite lateral faces, are wn- 
equal and rectangular in their intersections; and, at the sare 
time, each is unequal to the vertical. There are hence three 
unlike axes making rectangular intersections; and in allusion 
to the three unlike axes, the system is called the Zrimetric sys- 
tem. It is also named, in allusion to its including erect prisms 
having a rhombic base, the Orthorhombic system, orthos, in 
Greek, signifying straight or erect. 
This rhombic prism is represented in fig. 4. It has a rhom- 
bie base, like fig. 11; the lateral axes connect the centres of the 
opposite lateral edges; and hence they cross at right angles and 
are unequal, as in the rectangular prism. This right rhombic 
prism is therefore one in system with the right rectangular 
prism. 
Fig. 5 represents another rectangular prism, and fig. 6 
