6 A POPULAR EXPOSITION OF THE 
crystals. Crystals originate in almost all cases in which matter pas se 
from a gaseous, or liquid, into a solid state; but if the process take 
place too quickly, or the matter solidify without free space for expan- 
sion, crystalline masses, in place of regular crystals, will result. If a 
small fragment of arsenical pyrites, or native arsenic, be heated at one 
end of an open glass tube (five or six inches long and one-fourth of 
an inch in diameter), the arsenic, in volatilizing, will combine with 
oxygen, and form arsenious acid, which will be deposited at the other 
end of the tube, in the form of minute octahedrons (Fig. 3, below). 
In like manner, if a few particles of common salt be dissolved in a 
small quantity of water, and a drop of the solution be evaporated 
gently (or suffered to evaporate spontaneously) on a piece of glass, 
numerous little cubes and hupper-shaped cubical aggregations will re- 
sult. Boiling water, again, saturated with common alum, will deposit 
octahedral crystals on cooling: the cooled water not being able to 
retain in solution the full amount of alum dissolved by the hot water. 
In like manner, sugar, sulphur, and other bodies crystallise by slow 
cooling from the molten state. 
The study of crystal-forms constitutes the science of Crystallogra- 
phy. To enter into the details of this science would extend our 
present discussion beyond its proposed limits, and carry us altogether 
beyond the object in view—the simple determination of the names of 
commonly-occurring minerals—and hence we shall confine ourselves 
to the general statement, that crystals admit of being arranged in 
six groups, or “‘systems;” the forms of each individual group passing 
into one another by simple transitions, but having no relations to the 
forms of the other groups.* The names of these respective groups, 
® The reader desirous to take up the study of Crystallography in a more extended manner, 
may attend the author’s special courses of lectures which include that subject. In these, 
the use of crystallographic instruments is shewn, and the lectures are illustrated by nu- 
merous wood and porcelain models, drawings, and natural crystals. The following is ex- 
tracted from the syllabus of the advanced course on Mineralogy: 
CRYSTALLOGRAPHY, Par? I.—Crystals, how defined. Formation of Crystals. Elements 
of Crystals: planes, edges, angles; diagonals, axes. Forms and combinations. Replacing 
planes. General nomenclature of Forms and simple Crystals. Law of constant Angles. 
Measurement of Angles. Laws of Symmetry: Holohedral, Hemihedral, and Tetartohedral 
Forms. Classification of Crystals, Dimorphism. {[somorphism. Compound Crystals. 
Distortions. Pseudomorphs. 
Parr II.—The six systems of Crystallization. The Monometric system. The Dimetric 
system. The Hexagonal system. The Trimetric system. The Monoclinic system. The 
Triclinic system. Method of ascertaining the system of a given Crystal. 
Parr II1.—Optical and other physical relations of Crystallography. 
