Page:Elementary Text-book of Physics (Anthony, 1897).djvu/99

 '''69. Structure of Solids.'''—With the exception of organized bodies, all solids may be divided into two classes. The bodies of one class, which are characterized by more or less regularity of form, are called crystalline; those of the other class, exhibiting no such regularity, are called amorphous. For the formation of crystals a certain amount of freedom of motion of the molecules is necessary. Such freedom of motion is found in the gaseous and liquid states; and when crystallizable bodies pass slowly from these to the solid state, crystallization usually occurs. It may also occur in some solids spontaneously, or in consequence of agitation of the molecules by mechanical means, such as friction or percussion. Crystallizable bodies are called crystalloids.

Some amorphous bodies cannot, under any circumstances, assume the crystalline form. They are called colloids.

70. Crystal Systems.—Crystals are arranged by mineralogists in six systems.

In the first, or Isometric, system all the forms are referred to three equal axes at right angles. The system includes the cube, the regular octahedron, and the rhombic dodecahedron.

In the second, or Dimetric, system all the forms are referred to a system of three rectangular axes, of which only two are equal.

In the third, or Hexagonal, system the forms are referred to four axes, of which three are equal, lie in one plane, and cross each other at angles of 60°. The fourth axis is at right angles to the plane of the other three, and passes through their common intersection.

The fourth, or Orthorhombic, system is characterized by three rectangular axes of unequal length.

In the fifth, or Monoclinic, system the three axes are unequal. One of them is at right angles to the plane of the other two. The angles which these two make with each other, as well as the relative lengths of the axes, vary greatly for different substances.

In the sixth, or Triclinic, system the three axes are oblique to each other, and unequal in length.

71. Forces Determining the Structure of Bodies.—In view of