[1] _Proceedings of the Society for Psychical Research_, v. 486.
[2] "Philosophie der Geistes," Hegel's _Werke_, vii. 179, 406, 408
(Berlin, 1845). Cf. Wallace's translation (Oxford, 1894).
CRYSTALLITE. In media which, on account of their viscosity, offer
considerable resistance to those molecular movements which are necessary
for the building and growth of crystals, rudimentary or imperfect forms
of crystallization very frequently occur. Such media are the volcanic
rocks when they are rapidly cooled, producing various kinds of
pitchstone, obsidian, &c. When examined under the microscope these rocks
consist largely of a perfectly amorphous or glassy base, through which
are scattered great numbers of very minute crystals (microliths), and
other bodies, termed crystallites, which seem to be stages in the
formation of crystals. Crystallites may also be produced by allowing a
solution of sulphur in carbon disulphide mixed with Canada balsam to
evaporate slowly, and their development may be watched on a microscopic
slide. Small globules appear (globulites), spherical and non-crystalline
(so far as can be ascertained). They may coalesce or may arrange
themselves into rows like strings of beads--margarites--(Gr. [Greek:
margarites], a pearl) or into groups with a somewhat radiate
arrangement--globospherites. Occasionally they take elongated
shapes--longulites and baculites (Lat. _baculus_, a staff). The largest
may become crystalline, changing suddenly into polyhedral bodies with
evident double refraction and the optical properties belonging to
crystals. Others become long and thread-like--trichites (Gr. [Greek:
thrix, trichos], hair)--and these are often curved, and a group of them
may be implanted on the surface of a small crystal. All these forms are
found in vitreous igneous rocks. H. P. J. Vogelsang, who was the first
to direct much attention to them, believes that the globulites are
preliminary stages in the formation of crystals.
Microliths, as distinguished from crystallites, have crystalline
properties, and evidently belong to definite minerals or salts. When
sufficiently large they are often recognizable, but usually they are so
small, so opaque, or so densely crowded together that this is
impossible. In igneous rocks they are usually felspar, augite,
enstatite, and iron oxides, and are found in abundance only where there
is much uncrystallized glassy base; in contact-altered sediments, slags,
&c., microlithic forms of garnet, spinel, sillimanite, cordierite,
various lime silicates, and many other substances have been observed.
Their form varies greatly, e.g. thin fibres (sillimanite, augite), short
prisms or rods (felspar, enstatite, cordierite), or equidimensional
grains (augite, spinel, magnetite). Occasionally they are perfectly
shaped though minute crystals; more frequently they appear rounded
(magnetite, &c.), or have brush-like terminations (augite, felspar,
&c.). The larger microliths may contain enclosures of glass, and it is
very common to find that the prisms have hollow, funnel-shaped ends,
which are filled with vitreous material. These microliths, under the
influence of crystalline forces, may rank themselves side by side to
make up skeleton crystals and networks, or feathery and arborescent
forms, which obey more or less closely the laws of crystallization of
the substance to which they belong. They bear a very close resemblance
to the arborescent frost flowers seen on window panes in winter, and to
the stellate snow crystals. In magnetite the growths follow three axes
at right angles to one another; in augite this is nearly, though not
exactly, the case; in hornblende an angle of 57 deg. may frequently be
observed, corresponding to the prism angle of the fully-developed
crystal. The interstices of the network may be partly filled up by a
later growth. In other cases the crystalline arrangement of the
microliths is less perfect, and branching, arborescent or feathery
groupings are produced (e.g. felspar, augite, hornblende). Spherulites
may be regarded as radiate aggregates of such microliths (mostly felspar
mixed with quartz or tridymite). If larger porphyritic crystals occur in
the rock, the microliths of the vitreous base frequently grow outwards
from their faces; in some cases a definite parallelism exists between
the two, but more frequently the early crystal has served merely as a
centre, or nucleus, from which the microliths and spherulites have
spread in all directions. (J. S. F.)
CRYSTALLIZATION, the art of obtaining a substance in the form of
crystals; it is an important process in chemistry since it permits the
purification of a substance, or the separation of the constituents of a
mixture. Generally a substance is more soluble in a solvent at a high
temperature than at a low, and consequently, if a boiling concentrated
solution be allowed to cool, the substance will separate in virtue of
the diminished solubility, and the slower the cooling the larger and
more perfect will be the crystals formed. If, as sometimes appears, such
a solution refuses to crystallize, the expedient of inoculating the
solution with a minute crystal of the same substance, or with a similar
substance, may be adopted; shaking the solution, or the addition of a
drop of another solvent, may also occasion the desired result.
"Fractional crystallization" consists in repeatedly crystallizing a salt
so as to separate the substances of different solubilities. Examples are
especially presented in the study of the rare-earths. Other conditions
under which crystals are formed are given in the article
CRYSTALLOGRAPHY.
CRYSTALLOGRAPHY (from the Gr. [Greek: krystallos], ice, and [Greek:
graphein], to write), the science of the forms, properties and structure
of crystals. Homogeneous solid matter, the physical and chemical
properties of which are the same about every point, may be either
amorphous or crystalline. In amorphous matter all the properties are the
same in every direction in the mass; but in crystalline matter certain
of the physical properties vary with the direction. The essential
properties of crystalline matter are of two kinds, viz. the general
properties, such as density, specific heat, melting-point and chemical
composition, which do not vary with the direction; and the directional
properties, such as cohesion and elasticity, various optical, thermal
and electrical properties, as well as external form. By reason of the
homogeneity of crystalline matter the directional properties are the
same in all parallel directions in the mass, and there may be a certain
symmetrical repetition of the directions along which the properties are
the same.
When the crystallization of matter takes place under conditions free
from outside influences the peculiarities of internal structure are
expressed in the external form of the mass, and there results a solid
body bounded by plane surfaces intersecting in straight edges, the
directions of which bear an intimate relation to the internal structure.
Such a polyhedron ([Greek: polys], many, [Greek: hedra], base or face)
is known as a crystal. An example of this is sugar-candy, of which a
single isolated crystal may have grown freely in a solution of sugar.
Matter presenting well-defined and regular crystal forms, either as a
single crystal or as a group of individual crystals, is said to be
crystallized. If, on the other hand, crystallization has taken place
about several centres in a confined space, the development of plane
surfaces may be prevented, and a crystalline aggregate of differently
orientated crystal-individuals results. Examples of this are afforded by
loaf sugar and statuary marble.
After a brief historical sketch, the more salient principles of the
subject will be discussed under the following sections:--
I. CRYSTALLINE FORM.
(a) Symmetry of Crystals.
(b) Simple Forms and Combinations of Forms.
(c) Law of Rational Indices.
(d) Zones.
(e) Projection and Drawing of Crystals.
(f) Crystal Systems and Classes.
1. Cubic System.
2. Tetragonal System.
3. Orthorhombic System.
4. Monoclinic System.
5. Anorthic System.
6. Hexagonal System
(g) Regular Grouping of Crystals (Twinning, &c.).
(h) Irregularities of Growth of Crystals: Characters of Faces.
(i) Theories of Crystal Structure.
II. PHYSICAL PROPERTIES OF CRYSTALS.
(a) Elasticity and Cohesion (Cleavage, Etching, &c.).
(b) Optical Properties (Interference figures, Pleochroism,
&c.).
(c) Thermal Properties.
(d) Magnetic and Electrical Properties.