CRYSTALLOGRAPHY. A crystal is a piece of matter that, by the action of molecu lar forces, has assumed a definite geometrical form of some kind, with plane faces. There is a great variety of crystalline forms, each forth being characteristic of one or more substances; and C. is the science which classifies the forms and shows the rela tions that subsist among them. The great majority of substances are 7apable of under going the process of crystallization, the exceptions being principally complex organic substances which tend to assume a globular or spherical form approaching that of organized structures. The most favorable condition for the crystallization of any sub stance is from its solution in water or other liquid. A liquid usually dissolves more of a salt when warm than when cold; and when a warm saturated solution is allowed to cool, a portion of the salt deposits itself in crystals. This process is that which is gen erally followed in the crystallization of saline substances. A second process resorted to in the case of the metals, such as bismuth, antimony, etc., and sulphur, is to fuse the material in a vessel, and when it is cooled down, so as partially to solidify the mass, the crust is broken through, and the liquid still remaining is poured off, when a net-work of crystals is obtained. A third method is to vaporize the substance, which on con densation resolves itself into crystals. Examples of this class are the formation of snow crystals from the water-vapor in the atmosphere, and the minute black crystals of iodine obtained by allowing its vapor to condense in a cold vessel or on a cold surface.
Many circumstances affect the crystallizing power of substances. Thus, water may be cooled down below its freezing or crystallizing point (32° F.), provided it be kept perfectly still, without becoming solid; but on subsequent agitation, it instantly crystallizes. Similarly, a hot saturated solution of sulphate of soda, or glauber salt, if cooled down in a still place, does not crystallize, but immediately does so when the liquid is agitated, or a fragment of any solid substance is introduced into it. The size of the crystals obtainable from any fluid depends much on the rate of cooling, and the state of commotion of the liquid. The more slowly the solution cools down, and the more quietly the process of crystallization is allowed to proceed, the larger arc the crystals obtained; whilst, when the liquid is rapidly cooled, and agitation is kept up, the crystals are comparatively small, and generally not completely formed. The reason
of this will be at once apparent, for a large crystal is constructed of a multitude of smaller crystals, built up regularly so as to constitute a compound crystal of the same form as the more minute crystalline atoms; and when a liquid is cooled slowly in a state of rest, only a few minute crystals are produced at first, and these are gradually built round on all sides by successive layers, till large, well-defined crystals are the result; while, when the liquid is rapidly lowered in temperature, and especially when agitation is kept up, numerous minute crystals are formed at once, and do not adhere together. In either case, the liquid from which the crystals have separated is called the mother-liquor, and is a saturated solution of the salt.
The external forms of crystals amount to several thousands, but they may all be regarded as belonging to six different systems.
The regular system (otherwise called the cubic, octahedral, tesseral, tessular, spheroidal, or equi-axed system) is characterized by having three axes or straight lines passing through the same point, of equal lengths, and placed at right angles to each other. The best illustra tion of this system is the cube or hexahedron, which has six square faces or planes, and the three equal axes terminate in the center of each of the square faces. The planes, or squares are symmetrically arranged, so that each is perpendicular to one axis, and parallel to the other two. The crystals have each six square faces, with twelve equal edges, and eight equal angles. Examples of substances which crystallize in the form of the cube or hexahedron, are—common iron pyrites (FeS,), common salt, or the chloride of sodium (\a01), fluor spar (CaF), galena, or the sulphuret of lead (PbS), and the metals gold, silver, platinum, and copper. Another important crystalline form belonging to the regular system is the octohcdron, where the terminations of the axes are in the angles of the crystals. It has eight faces, all of which are equilateral triangles, and twelve edges, with six angles, each of which has four faces. The diamond (C), alum, ziac Mende (ZnS), sal ammonia C. magnetic iron ore (Fe,04), fluor spar (CaF), and chrome iron ore are examples. There are various secondary forms belonging to this system, derivable from the cube and octohedron, such as the rhombic dodecahedron, which has twelve faces, and is the form in which the garnet crystallizes.