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phenomena of Decrepitation (Zerknistern), the vapour given off from the mother-liqnid bursting the crystalline mass with violence. This water of decrepitation which, as an accidental mechanical admixture has no influence on the form of the crystal, is altogether different and must be carefully distinguished from the cheniically-combined water which certain crystals contain in definite proportion, and which is essential to their crystalline form. Common salt crystallized by slow evaporation from an aqueous solution does not decrepitate; but when crystallized by rapid boiling of the liquid it decrepitates violently. Many forms of calcspar decrepitate, others not. According to Baudrimont (J. Pharm. 22, 337), who regards decrepitation as the result of unequal heating of laminar masses (?), the salts which exhibit this property most strikingly are: iodide, bromide and chloride of potassium, bromidle and chloride of sodium, sulphate, chromate and bichromate of potash, nitrate of baryta, red ferrocyanide of potassium and cyanide of mercury.

Decrepitation takes place for the most part in those crystals obtained from aqueous solutions, which do not contain chemically combined water: some salts, however, which contain small quantities of combined water, such as bicarbonate of potash, bitartrate of potash, tartar-emetic, and neutral acetate of copper, decrepitate slightly from containing mother-liquid mechanically inclosed in them. The occurrence of decrepitation in chloride of sodium artificially crystallized from an aqueous solution, and its non-occurrence in rock-salt, leads to the supposition that the latter has been crystallized from a state of igneous fusion. There is but one variety of rock-salt that decrepitates when heated, viz., the decrepitating salt of Wielitzka, the decrepitation proceeding however not from mechanically included water, but from hydrogen and other inflammable gases existing in the salt in a high state of condensation. A similar phenomenon is observed in many crystallized minerals occurring in metallic veins, such as heavy spar, calcspar, fluor spar, ironspar, lead spar, galena, iron pyrites, copper pyrites, grey copper ore, &c., which sometimes decrepitate, sometimes not; and in the former case give out, not water, but probably a gas which was enclosed between their lamina in a highly compressed, perhaps even in the liquid state. (H. Rose, Pogg. 48, 354.)

If a solution, in addition to the crystallizing substance, likewise contains others which are less easily crystallizable, the latter will remain in the mother-liquid after the separation of the greater part of the former. This circumstance furnishes a method of purifying easily crystallizable substances by repeated solution, crystallization, pouring off of the mother-liquid, washing with small quantities of the cold solvent, and pressing between blotting-paper.

In this method of purification, the formation of large crystals by slow cooling or evaporation is usually preferred, because they present fewer surfaces, and are therefore more easily freed by washing from the adhering mother-liquid. On the contrary, in the new French method of purifying saltpetre, the smallest possible crystals are formed by constant stirring and rapid cooling of the hot solution, because large crystals of this salt contain a larger quantity of mechanically included mother-liquid which cannot be removed by washing. If however it be remembered that the more slowly crystallization takes place, the less is the quantity of mother-liquid inclosed, and moreover that small crystals are much more difficult to free from liquid adhering to their surfaces than large ones, we shall probably agree with Clement and Désormes (Ann. Chim. 92, 248), notwithstanding the objections of Long

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(3.) All

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champ (Ann. Chim. Phys. 9, 200) in giving the preference to slow crys-
tallization. The methodical purification of a salt, saltpetre for example,
by crystallization, may be conducted as follows. (1.) The saltpetre is
heated with water in a vessel A, till it is dissolved. It is then left to
crystallize by cooling, the mother-liquid poured off as completely as pos-
sible into another vessel B, by gradually inclining the vessel A; the crys-
tals are then repeatedly washed with a small quantity of cold water,
which after such washing is run off as completely as possible into B.
(2.) The vessel a is heated with water till the salt is dissolved, and B
till { of the contained liquid is evaporated; the mother-liquid from B,
after the crystals are deposited, is poured into a third vessel c; the crys-
tals in B repeatedly washed as aforesaid; then the mother-liquid and like-
wise the wash-water from a poured upon the crystals in B.
three vessels are heated till the crystals in A are dissolved in fresh water,
those in B in the mother-liquid and wash-water from A, and the liquid in c
sufficiently concentrated. In this manner the process is continued, smaller
and smaller vessels being used in the successive stages till the saltpetre in
A appears perfectly pure, then that in B, and so on Nearly all the im-
parities of the saltpetre collect in the last vessel a; those near it contain
small quantities of the salt in an impure state.

5. The formation of crystals is always accompanied by development
of heat: this is particularly evident when the crystallization is rapid, and
is therefore most conspicuous in the before-mentioned anomalous cases of
the sudden crystallization of a cooled liquid. In a few instances crystal-
lization is attended with production of light. (vide Light.)

External Form of Crystals.
The number of crystalline forms amounts to several thousands.
According to the proportions of their linear dimensions, or axes, they may
be arranged in a small number of groups, which in Weiss's System of
Crystallography (Abh. d. physik. Classe der K. Academie d. Wiss. zu
Berlin, 1814, 1815, S 298), are as follows:-

A. The proportion of the parts may be determined by three linear
dimensions or axes, at right angles to each other.

a. The three dimensions equal. Regular, Spheroidal, Tessular system. (Reguläres, sphäroëdrisches, tessularisches krystallsystem)

a. All the faces of the crystal similar; at each end of the axis the relation between the faces in four directions is the same. Homospheroidal system (Homosphäroëdrisches system) to which belong the cube, regular octohedron, rhomboidal dodecahedron, trapezohedron, pyramidal cube, pyramidal octohedron, pyramidal dodecahedron, &c. (Diamond, salammoniac, common salt, sulphuret of zinc, most simple metals, &c. Fig. 1-12.)

B. One half of the similar faces is obliterated by the other half: at each end of the axes a similar relation of the faces exists in two opposite directions only. Hemispheroidal system (Hemisphäroëdrisches system). To this belongs, on the one hand, the regular tetrahedron formed by the obliteration of four faces of the regular octohedron, with its modifications (Fablerz, boracite, fig. 13—17): and, on the other hand, the pentagonal dodecahedron and its modifications, formed by the obliteration of 12 faces of a pyramidal cube. (Iron pyrites, fig. 18—20.)

b. Only two axes equal, Four-membered, Two and one-axis, or Square prismatic System (Viergliedriges, zwei-und einaxiges oder quadratisches system), to which belong the acute and obtuse ociohedron with a square

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base, the square prism, and the regular octagonal prism (Molybdate of lead, zircon, calomel, ferrocyanide of potassium, tin-stone, &c. Fig. 21–40.) c. All three axes unequal.

All the similar faces exist: the primitive form to which the others may be immediately reduced is an octohedron with a rhombiç base. Two and two-membered, One and one-axis, or Right prismatic system (Zwei and zwei-gliedriges oder ein und ein-axiges system). This comprises the rhombic octobedron (fig. 41), the rectangular octohedron (fig. 47), two double tetrahedrons (one represented in fig. 54), the upright rhombis prism (fig. 44), the rectangular prism, and numerous more complicated forms. (Sulphur, nitrate of potash, sulphate of lead, heavy spar, sulphate of magnesia, &c. Fig. 41-80.)

B. Part of the similar faces obliterated; the lower fore and upper back y-surfaces of fig. 54. The primitive form is a rhombic prism with oblique terminal faces, so placed that one diagonal of the terminal faces. is perpendicular to two of the lateral edges, the other diagonal inclined to the remaining edges; sometimes the base is obliquely inclined to the obtuse lateral edges (fig. 81), sometimes to the acute ones (fig. 91). To this system also belongs the oblique rectangular prism (fig. 82 and 92). Two and one-membered or Oblique.prismatic system (Zwei und eingliedriges system). (Augite, borax, gypsum, phosphate of ammonia, green vitriol, &c., fig. 81–119.) A modification of this system is the one and two-membered (ein-und zweigliedriges system).

We must here introduce the system discovered by Mitscherlich (Pogg. 8, 427) in hyposulphite of lime and protonitrate of mercury, the primitive form of which is a rhombic prism, in which the faces u and ú have the same value, but the base is obliquely inclined to all the lateral edges: as happens, for example, when the face a (fig. 99) obliterates the faces i and á, (fig. 120.)

d. By obliteration of two parallel y-faces, and two parallel-u-faces (fig. 54) at once, and replacing of the latter by two new faces V, there arises a rhomboidal prism whose terminal faces are obliquely inclined in such a manner that both their diagonals make oblique angles with the lateral edges of the prism. Only the parallel faces and the diagonally opposed edges and summits are equal and similar. One and one-membered or Doubly oblique prismatic system (Ein-und eingliedriges system). Sulphate of copper, boracic acid, gallic acid, &c. Fig. 121—130.

B. The relation between the parts of the crystal may be determined by assuming 4 linear dimensions, 3 of which, of equal length, are contained in one plane and inclined to one another at angles of 60°: the fourth is placed at right angles to them. Three and one-axis or Rhombohedral system (Drei-und einariges system).

a. All the similar faces exist: at each end of the lateral axes, the same relation of the faces exhibits itself both above and below. Sixmembered system (Sechs-gliedriges system) including the double six-sided pyramid, the six-sided and twelve-sided prism, &c. (Apatite, &c. fig. 131-140.)

6. Of each pair of similar and parallel faces one is wanting; at each end of the lateral axes

the
upper
face

faces differ from the lower both in number and situation. Three and three-membered system (Drei-und dreigliedriges system). The primitive form of this system is the rhombohedron, wbich we may imagine to be formed from the double six-sided pyramid by the obliteration of half of the faces alternately disposed.

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(Calcspar, cinnabar, peroxide of iron, sesquioxide of chromium, &c., fig. 141-160*.)

The primitive forms of these systems are moreover subject to innumerable modifications from truncations, formation of edges and angles, divisions and curvatures.

The same kind of matter may crystallize in various forms, which however in most cases belong to one single crystalline system only, are compatible with regard to their angles, and may be deduced from a common primitive form. Thus, calcspar occurs in more than 100 crystalline forms which however all belong to the 3 and 3-membered system, and are derivable from an obtuse rhombohedron (fig. 141). If we are acquainted with but one form of a crystalline body we may yet conclude that the body might, under certain circumstances, assume all the other forms belonging to the same system. Why the same substances should assume sometimes one sometimes another form belonging to the same system, is not yet satisfactorily ascertained. According to Beudant (Ann. Chim. Phys. 8, 5), temperature, electrical condition, the concentration and volume of the liquid, the form and substance of the containing vessel, the state of the barometer and hygrometer, have no influence on the form assumed. Bouchardat also (Ann. Chim. Phys. 52, 296), obtained common salt constantly in cubes, and alum in octohedrons, differing only in magnitude, whether the crystallization took place in vessels of sulphur, graphite, or metals of the most various kinds. The greatest influence appears to be exerted by the presence of foreign bodies in the crystallizing liquid. Sal-ammoniac, which crystallizes in octohedrons from a solution in pure water, produces cubes when the liquid contains a large quantity of urea, and cubo-octohedrons when a small quantity of urea or boracic acid is present. Common salt, which when alone crystallizes in cubes, assumes the octohedral form when the liquid also contains urea, and the cubo-octohedral when boracic acid is present. Chloride of potassium which separates in cubes from a pure aqueous solution, deposits cubes with truncated edges when the liquid also contaius corrosive sublimate (fig. 5). A solution of alum, to which a little alcohol has been added, yields cubes instead of octohedrons; the addition of hydrochloric acid to the same solution causes the alum to crystallize in cubo-icosahedrons (fig. 20); the addition of borax gives rise to cubo-octo-dodecahedrons (fig. 8). Protosulphate of iron, which by itself crystallizes in the form shown in fig. 111, yields, according to Beudant, when its solution is mixed with sulphate of zinc or sulphate of magnesia, crystals which exhibit only the i-, u- and c. faces; but if hydrochloric acid, borax, or

* Crystallographical Nomenclature of different writers :Weiss.

Mons. NAUMANN. BREITHAUPT. HAUSMANN.

tessularisch
pyramidal

tesseral
tetragonal

tessularisch
tetragonal

isometrisch
mono-dimetrisch

Regulär
4 gliedrig oder
2 u. 1 axig.
2 u. 2 gliedrig

oder
1 u. 1 axig.
2 u. 1 gliedrig

orthotyp

rhombisch

holoedrisch
rhombisch

prismatisch

1 u. 1 gliedrig

hemiarthotyp mono-klinoedrisch hemiedrisch trimetrisch

rhombisch anorthotyp tri-klinoedrisch tetartoedrisch

rhombisch rhomboedrisch hexagonal hexagonal mono-trimetrisch Mitscherlich's Syst. hemianorthotyp diklinoedrisch.

3 1. I axig.

VOL. 1.

C

phosphate of soda be present, the crystals exhibit a greater number of faces than those represented in fig. 111. According to Beudant again, a solution of alum or green vitriol mixed with finely pounded sulphate of lead, deposits in the paste which settles at the bottom a number of crystals having fewer and less polished faces than those obtained from pure solutions of the same salts: this effect is attributed by Beudant not so much to any mechanical influence exerted by the powder, as to the chemical action of the extremely small quantity of sulphate of lead which dissolves in the water. The peculiar forms of fluor-spar mentioned on page 13, likewise indicate the presence of foreign matter at certain times during its crystallization. In some cases only, as for example that of protosulphate of iron mixed with sulphate of zinc or sulphate of copper, has it been demonstrated that the foreign body separates from the solution along with the crystals; in most instances, on the contrary, e.g., common salt with urea, this does not appear to be the case, and the action of these substances is perhaps for the most part attributable to the fact that their presence in the liquid from which the body crystallizes occasions the union of its particles according to fixed laws.

The rule that all the crystalline forms of any particular substance belong to the same system, and may be derived from the same ultimate form is subject to several exceptions; many substances, both simple and compound, are dimorphous and perhaps even trimorphous, i.e., they present according to circumstances, 2 or 3 different groups of crystalline forms, which may be reduced to 2 or 3 different systems, or at least to 2 or 3 primitive forms. (Vid. Affinity.)

The number of systems of crystalline forms being small, while that of crystallizable bodies amounts to several thousands, it necessarily happens that a great number of bodies, differing widely in other respects come under one crystalline system; and, on the other hand, detinite individual forms of the same system are found belonging to bodies of very different nature. If the forms of different substances belong to the regular system, there can be no difference in the magnitude of their

angles, because the 3 axes of that system are equal: thus the angles of the regular octohedron remain the same, whether the crystals consist of alum, sal-ammoniac, or diamond. In the other systems, on the contrary, since the axes are unequal

, and the inequality is of different magnitude in different substances, it follows that angular differences of various amount must exist in the crystals belonging to different bodies included in these systems. Thus the octohedron with a square base of Anatase (fig. 21), is acute, that of Zircon (fig. 23) obtuse, because in the former the longitudinal axis is longer, in the latter shorter than the lateral axes. These angular differences, however, are often very small: thus, for the blunt lateral edge of the rhombic prism of sulphate of magnesia, we find (u : u' fig. 71) 90° 30'; in sulphate of zinc (u : u fig. 73) 91° 71: and the angle of the terminal edges of the obtuse rhombohedron (" : 7') amounts in calcspar to 105° 5', in manganese spar to 106° 51', in iron spar to 107° 2', in bitter spar to 107° 22', and in calamine tó 107° 40'. This near approach to equality in the angles is, however, often co-existent with similarity of chemical constitution. (Vid. Isomorphism, under the head of Affinity.) i

Internal Structure, Texture of Crystals. Almost all crystals may be more easily split or cloven, in certain directions lying in determinate planes, than in others; they exhibit from

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