process involves the destruction of the original compound, whereby products of simpler composition are obtained. In order to avoid oxidation, destructive distillation must be carried on in closed apparatus with entire exclusion of air, and as the heat necessary is in most cases far greater than that to which glass vessels could be safely exposed, iron retorts or cylinders are employed. The residue left in the iron retort is often a fused mass insoluble in water, which necessitates mechanical means for its removal. The products of destructive distillation, in their crude state, are usually accompanied by a peculiar smoky odor called empyreuma, said to be due to an oil developed during the process of decomposition; this is subsequently removed by rectification. The most striking examples of destructive distillation are the manufacture of acetic acid from wood and of illuminating gas from coal.

Sublimation.

Sublimation is the term applied to the process of vaporizing volatile solids and condensing the vapor back into a solid; it must not be confounded with the term dry distillation, which is frequently used in place of destructive distillation. The product of sublimation is known as a sublimate, and may occur either in the form of a fine powder or compact masses.

The object of the process of sublimation may be the purification of a substance by separating the volatile solid from less volatile or fixed impurities, as in the case of sulphur, camphor, naphtalene, and iodine, or the separation and collection of volatile solids resulting from chemical reaction at higher temperatures, as in the case of pyrogallol, calomel, and mercuric chloride.

The apparatus consists of a subliming vessel made of iron, glass, or earthenware, and a condenser adapted to the volatility of the product, the condensing surface being kept sufficiently near the source of heat to avoid cooling of the vapor before it reaches the condenser. If the temperature of the condenser is but little below that of the subliming vessel, the vapors of the volatilized substance will not condense until they strike the surface of the condenser, and will form in compact masses, frequently in crystalline condition; as for instance, arsenous acid, corrosive mercuric chloride, ammonium carbonate, and commercial sal-ammoniac. In order to obtain the sublimate in the form of powder, the air in the condenser must be decidedly cooler than the temperature at which the substance volatilizes, because then the vapor will be immediately cooled and rapidly deposited in very small particles, as in the case of calomel, sulphur, and camphor when intended for subsequent compression.

The process of sublimation is confined to the larger operations of the manufacturing chemists, but can be demonstrated in a small way by placing a few grains of camphor or iodine in a long test-tube and then heating until all has been volatilized; in a few minutes the substance may be gathered in the form of very small crystals from the upper part of the tube.

CHAPTER XI.

CRYSTALLIZATION.

THE subject of crystallization, while a most important branch of mineralogy and chemical physics, is of less value in pharmacy proper; but, as the Pharmacopoeia makes frequent use of terms belonging to the study of crystallology, and as the pharmacist may have occasion to resort to crystallization for the purpose of determining the character and quality of substances, a short notice is deemed

desirable.

FIG. 173.

a

f

b

Crystallization may be looked upon as another method of separation, as it is frequently employed for the purpose of removing impurities from crystallizable substances. The term crystal is applied to solid inanimate bodies of regular internal structure and definite geometrical form, bounded by plane surfaces and having angles of fixed and constant values. assumption of such distinctive geometrical forms occurs, as a rule, during the change taking place in the state of aggregation of substances from the gaseous or liquid to the solid condition; in a few cases it occurs also in solid bodies. as iron and brass wire.

g

-E

h

-k

'd

The

In the preliminary study of crystallography, the meaning of the following terms must be considered.

Faces are the plane surfaces bounding the crystal (see abde, efhg, abfe and bfhd, Fig. 173).

Edges are the lines of intersection of two adjoining faces (see ef, ab, fh, bf, db, eg, ea, gh, gf, cd, ca, cg, etc., Fig. 173).

Angles are the points formed by intersection of three or more faces (see Fig. 173), e, formed by abef, eacg, and efhg; f, formed by bdhf, baef, and efgh; c, formed by dhge, abde, and aegc, etc.

Axes are imaginary lines drawn through the centre of the crystal, around which the symmetrical deposit of matter has occurred during the formation of the crystal (see ik, lm, and no, Fig. 173). Amorphous (without form) designates the absence of crystalline form and structure, as in acacia, starch, gelatin, etc.

Di- or tri-morphous (of two or three forms), indicates that the sub

stance occurs in two or three distinct crystalline forms, as carbon, sulphur, etc.

Polymorphous means of many forms.

Isomorphous (of the same form) indicates that two or more substances to which the term is applied, crystallize in the same form; thus the chlorides, iodides, and bromides of sodium and potassium are isomorphous. Isomorphous bodies are known to resemble each other also in chemical composition, and to permit of a ready interchange of constituents, as in the case of the various alums.

Cleavage is the tendency of most crystals to split in particular directions, affording usually even and frequently polished surfaces, the direction being always parallel with the planes of the axes, or with others diagonal to these. While some crystals cleave very easily, in others this tendency is scarcely discernible.

Tabular crystals are such as crystallize in flat plates, as potassium chlorate, iodine, strontium iodide, etc.

Laminar crystals are such as crystallize in thin plates, as acetanilid, naphtol, calcium hypophosphite, etc.

Acicular crystals are such as occur needle-shaped, as aloin, cinchonidine sulphate, quinine salts, etc.

Prismatic crystals are such as resemble a prism, being extended chiefly in the direction of the longest axis, as salicylic acid, santonin, cinchonine sulphate, etc.

Orthometric refers to the measurement of the angles, and is used to signify that the three axes intersect each other at right angles. Clinometric refers to the intersection of the axes at oblique angles. Holohedral, applied to crystalline forms, signifies that the full number of faces required by perfect symmetry are present.

Hemihedral signifies that only one-half the number of faces required by full symmetry are present.

Crystals are formed according to fixed laws of Nature, and there can be no doubt that the force of cohesion plays an important part in their formation; but no one knows how, or why, the molecular particles of certain substances arrange themselves into symmetrical deposits, around a common centre, in a manner to give rise to numerous distinct and definite forms.

The large variety of forms in which crystals appear, depends entirely upon the number and length of the axes and their relative inclination—that is, the angles at which they intersect each other. All crystalline forms have been reduced by scientists to two main groups, the orthometric and the clinometric groups (see above), and these have again been subdivided into six systems; the orthometric group comprises the regular, quadratic, rhombic and hexagonal systems; the clinometric group, the monoclinic and triclinic systems. As all crystals belong to one or the other of these systems, the salient features of each should be studied.

1. The Regular System, also known as the Monometric, Cubic, Octohedral, or Tessular System.

Crystals of this system have three axes of equal length, which intersect each other at right angles, as shown in Fig. 174.

The fundamental forms of this system are the cube and the octohedron, Figs. 175 and 176.

Alum, phosphorus, arsenic trioxide, diamonds, alkali iodides, chlorides, fluorides and cyanides, as well as many metals and their sulphides, crystallize in this system.

2. The Quadratic System, also known as the Dimetric, Square Prismatic, or Tetragonal System.

Crystals of this system have three axes intersecting each other at right angles, two of which are of equal length, and one either longer or shorter than the other two; the two equal axes are called secondary axes, while the third is termed the primary axis. See Fig. 177.

The fundamental forms of this system are the quadratic octo

hedron (also called square-based double pyramid) and the rightsquare prism, Figs. 178 and 179. The pyramids of this system have square bases.

FIG. 178.

Quadratic octohedron.

FIG. 179.

Right square or quadratic prism,

Among the modified forms are the truncated quadratic octohedron, Fig. 180, and the quadratic pyramidal prism, Fig. 181.

FIG. 180.

FIG. 181.

Truncated quadratic octohedron.

Quadratic prism with pyramidal ends.

Potassium ferrocyanide, calomel, nickel sulphate, boron, tin, stannic oxide, magnesium sulphate, zinc sulphate, etc., crystallize in this system.

3. The Rhombic System, also known as the Trimetric or Right Prismatic System.