CHAPTER XV.

CRYSTALLIZATION.

CRYSTALLIZATION is the process whereby substances are caused to assume certain determinate forms called crystals. These are distinctive, and when perfect are bounded by geometrical surfaces. Those substances which are not crystallizable are termed amorphous. The objects of the process are to increase the purity and to enhance the beauty of chemical substances. The descriptions of the crystalline forms assumed by bodies form the basis of the interesting science of crystallography. In a work of this kind it is impossible to give more than a very brief sketch of the outlines of the classification, since the practical process of crystallization must receive the most attention. (See Dana's Mineralogy, Kopp's Krystallographie, Miller's Mineralogy, etc.) Every crystallizable body invariably assumes its own characteristic form, or some form directly derived from it or related to it by a simple law, and in order to classify them crystallographers recognize at the present time six systems, to one or other of which every crystal is referred. A seventh system is sometimes conceded, but the occurrence of crystals belonging to it has not been demonstrated with certainty.

The following definitions should be well understood: The plane surfaces bounding a crystal are termed faces; when two contiguous faces intersect, an edge is formed; an angle is formed when three or more faces intersect.

The faces, edges, or angles of a perfect crystal have equal faces, edges, or angles opposite to them, and if the middle point of the opposite faces or edges, or the opposite angles, be joined by straight lines, the point at which these lines intersect will be the centre of the crystal. The lines drawn through this point are called axes.

When the same body crystallizes in two or more forms belonging to different systems, it is said to be dimorphous, trimorphous, polymorphous, etc. When different substances crystallize in the same form, they are said to be isomorphous.

Prismatic (or prism-like) crystals are those which are extended principally in the direction of their longest axis. Tabular crystals are those crystallizing in flat plates; laminar, those crystallizing in thin plates; acicular, those which are needle-shaped, etc.

Other terms are used to describe the physical characters of crystals, which are readily understood and are not technical in their meaning.

The systems of classification are based upon the length and relative position of the axes of the crystal. Those in which the three axes intersect at right angles are termed orthometric; and when the angles caused by their intersection are oblique, they are called clinometric.

SYSTEMS IN CRYSTALLOGRAPHY.

I. Monometric, or Regular System.-The crystals have three axes of equal length intersecting at right angles (see Figs. 286, 287, and 288).

two of which are equal, the other different in length, all intersecting at right angles (see Figs. 289 and 290).

III. Trimetric, or Rhombic System.-The crystals have three axes

of unequal length, all intersecting at right angles (see Figs. 291 and 292).

IV. Hexagonal, or Rhombohedric System.-The crystals have four axes, three of equal length, in the same plane, and inclined to one another at angles of 60°. The fourth axis is different in length, and intersects the plane of the other three at right angles (see Figs. 293 and 294).

V. Monoclinic, or Oblique-Prismatic System.-The crystals have three axes of unequal length, two of which are obliquely inclined to

each other, the other axis forming right angles with these two (see Figs. 295 and 296).

VI. Triclinic, or Doubly-Oblique Prismatic System.-The crystals

have three axes of unequal length, all obliquely inclined to one another

(see Figs. 297 and 298).

The Diclinic System, if recognized, would have three axes, two at right angles to each other, the third oblique to the other two.

Determination of Crystalline Form.-The method of determining the position of a crystal in one or other of the systems above noted is to measure the inclination of the angles which the faces of the crystal make with one another. From the data obtained the length and inclination of the axes are calculated. The hand goniometer or Wollaston's reflecting goniometer is used to measure the angles.

Cleavage. If a crystal of potassium ferrocyanide or a piece of mica is broken and examined, it will be noticed that the cohesion of the particles is less in one direction than in any other, and if the blade of a knife is inserted in the edge, the crystal may be easily split or cleft. Other crystals possess this property, but to a greatly varying extent. crystals may sometimes be formed by cleavage.

Perfect

The Process of Crystallization generally takes place when a body passes from a liquid or a gaseous condition into the solid state: a few instances are known where amorphous solids become crystalline without becoming liquefied, as in iron or brass wire, sulphur, barley-sugar.

Methods of Obtaining Crystals.-1. By fusion and partial cooling. 2. By sublimation. 3. By deposition from supersaturated solutions as they cool. 4. By deposition from solutions during evaporation. 5. By deposition from solutions upon passing through them a galvanic current. 6. By precipitation. 7. By the addition of a substance having a strong affinity for water.

1. By Fusion and Partial Cooling.-Substances which have low melting-points, like sulphur, camphor, and iodine, and some of the metals, like bismuth, antimony, etc., may be crystallized in this way. To obtain crystals of a substance like sulphur, it should be melted in a deep vessel and then allowed to cool, so that a crust will be formed; a hole is then made in the crust, and a smaller one on the opposite side; the vessel is now inclined towards the side having the larger hole, and the melted substance runs off; when the surfaces inside are examined, they will be found studded with crystals. If the quantity of material used is large, and the mass has been gradually cooled, the crystals will be large and distinct. The crust should be perforated as soon as it is fairly formed, and the fluid contents quickly removed.

2. By Sublimation.-This is one of the most useful methods of obtaining crystals (see Sublimation, page 171).

3. By Deposition from Supersaturated Solutions.-This is the method by far most frequently employed to obtain crystals. The solution of the substance is generally effected by the use of heat (see Solution): it should be carefully filtered, and evaporated to the proper degree, and this latter part of the operation is the most important in determining the size and beauty of the crystals. As a rule, concentrated solutions produce small, ill-defined crystals, whilst comparatively dilute solutions, provided they are supersaturated, produce crystals of more perfect form. The proper degree of concentration must always depend upon the solubility of the substance: if very soluble, the solution should not be saturated at the boiling temperature, or the crystals will be very small and so thoroughly interlaced that it will be difficult to wash them; if a por

tion of the evaporating solution is transferred to a glass or porcelain plate and allowed to cool, the rapidity with which the small quantity of solution crystallizes, and the amount of crystals obtained, form a basis for judgment. Upon the large scale, in order to secure a uniform product, it will be found that the specific gravity of the solution at a definite temperature, the temperature of the air, and the quantity of the solution must be considered: these points, however, can be obtained only by experience, and after a practical trial with each substance. It is a good habit to keep a record at each operation of the specific gravity and temperature of the solution which is set aside to crystallize, and note the character of the product. If the substance is not very soluble, the solution should be evaporated until a pellicle or crust is formed upon the top, and then set aside.

Perfect Rest for a solution designed for crystallization must be secured, if well-defined crystals are wanted, and the solution must not be cooled quickly. When small crystals are desired, as in the case of magnesium and zinc sulphate, the solution should be cooled quickly, with constant agitation: this produces a great many nuclei, and prevents the gradual deposition of the particles in regular order upon one nucleus, which is so essential to the formation of the perfect crystal. There are several plans to choose from, for preventing rapid cooling: if the liquid is placed in an evaporating dish, and heated in a sand-bath or water-bath until evaporated to the proper point, the whole may be set away without disturbing them, to cool slowly together; or the dish may be placed in a warm room which is slowly cooled; or it may be embedded in a blanket or in woollen cloths, covered, and set aside. Having arranged the dish, it must be left absolutely undisturbed until all the crystals have separated: if jarred or knocked after the crust has once formed, the crystals will be mere confused masses.

Use of Nuclei.-It has long been known that if a smooth glass rod having a single scratch upon it be placed in a solution ready to crystallize, crystals will first attach themselves to the scratched part, and the smooth part of the rod will frequently not have any separate crystals upon it. Rough surfaces, by offering more points of adhesion, attract the nuclei upon which the crystalline body is subsequently deposited: it is for this reason that strips of wood or lead are frequently suspended in liquids intended for crystallization, whilst in the manufacture of rockcandy, threads are usually strung across the crystallizing-tubs at regular intervals, columnar masses of fine crystals being thus produced. Perfect geometrical crystals may be obtained by the practice of "nursing, which consists in selecting from the ordinary stock as perfect a crystal as can be found for the nucleus, and then suspending it by a horse-hair or piece of sewing-silk in a warm saturated solution of the salt. Prof. J. U. Lloyd contributed to New Remedies, in 1879, pp. 98, 133, 162, some interesting notes on the production of perfect crystals.

Retarded Crystallization.-Warm saturated solutions of various salts, particularly if contained in chemically clean vessels, protected from the dust, and left at absolute rest until cooled, usually fail to crystallize. If the receptacle is shaken or jarred, or if a crystal from which the solution has been made, or any other solid substance, is dropped into it, crystal