(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 contains 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:

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, definite 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. 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° 7': and the angle of the terminal edges of the obtuse rhombohedron (r: r') 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 to 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.)

Internal Structure, Texture of Crystals.

These

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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1 to 7 planes of cleavaye (Blatterdurchgänge) intersecting one another at determinate angles. This different facility of separation of a crystalline mass in certain directions may be shown not only by mechanical, but also, according to Daniell (Schw. 19, 38 u. 194) by chemical means; for when masses of different substances having a crystalline structure but no determinate external form, are placed in a liquid which does not act too rapidly on them, the undissolved portions sometimes exhibit grooves and depressions in the directions of the planes of cleavage, sometimes assume with tolerable distinctness the primitive forms of the systems to which the bodies belong. Again, when pieces of native sulphuret of antimony (Grauspiess-glanzerz) are placed in recently fused sulphuret of antimony, and half melted, the unmelted portion assumes the form of distinct crystals. (Faraday, Qu. Jour. of Sc., 1821: also Schw. 32, 481.) To this class of phenomena also belong the Figures of Widmanstadt, and the Moiré metallique, i. e., certain figures corresponding to the planes of cleavage, which come to light when meteoric iron or tin-plate is acted upon by acids. The more distinct planes of cleavage of a body are generally parallel to the faces of one of the primitive forms of the system to which it belongs: the less distinct to other less important faces of the same system. Thus, fluorspar has 4 planes of cleavage corresponding to the 8 faces of the regular octohedron, or the 4 faces of the regular tetrahedron; the 3 planes of cleavage of heavy spar (fig. 49) are parallel to the faces p, u, and u' of the upright rhombic prism; the 3 cleavage planes of calcspar to the 6 r- faces of the obtuse rhombohedron (fig. 141), &c. In different crystals of the same substance, one or other of the less distinct cleavage planes is often wanting; those however which can be traced, always make the same angle with each other, whatever may be the outward form of the crystal. Different substances may present the same cleavage planes when they belong to the regular system: if, however, they belong to any other system, they always exhibit at least slight differences in the directions of their cleavage planes. Imagine a crystal to be cloven according to its most distinct cleavage planes, or according to them all; it will then be resolved into the so-called Simple Molecules (Molécules intégrantes), whose form is either a regular or irregular tetrahedron, a regular or irregular three-sided prism, or a parallelopiped. When the faces of a crystal do not run parallel to its principal cleavage planes (the so-called Secondary Form), it is possible, by splitting the crystal at certain points, in directions parallel to these planes, to remove an external envelop, the so-called Secondary mass, and leave in the middle a crystalline kernel or Nucleus, whose faces are parallel to the principal cleavage planes. This form is regarded by Hauy (Traité de Minéralogie, T. 1), as the Primitive Form, which he supposes to have been developed first: he further supposes that on the faces of this primitive form there have been deposited successive laminae consisting either of simple molecules or of aggregations of the same into compound molecules (molécules soustractives); and that these, being deposited in such a manner that the dimensions of the lamina go on decreasing from one of the edges or summits, have produced the secondary form. This, however, is nothing more than a theoretical view, of which Hauy availed himself in calculating the arrangement of the secondary faces, since it is found that crystals on their first appearance exhibit the same form as after their complete development. Moreover Weiss has shown that independently of any such unnatural hypothesis, the angles of the different primitive and secondary faces of a crystal may be calculated from the mere proportion of its

linear dimensions. The atomic theory seeks to explain the structure of crystals by attributing a distinct form either to the atoms themselves, or if these be regarded as spheres, to aggregations of several of them. (Vid. Affinity.) The advocates of the dynamic theory proceed partly from the hypothesis that every solid body differs from a fluid in this respect, that the cohesion of its particles is of different amount in different directions, and further, that in a crystal these directions extend through the whole mass in straight polar lines.

ADHESION.

That kind of attraction which acts at infinitely small distances only between bodies of different natures, giving rise to the union of these bodies into a heterogeneous whole called a Mixture or Mechanical Combination, which may in most cases be overcome by mechanical force.

It appears to be exerted between all kinds of matter, imponderable as well as ponderable, but in various degrees. [On the adhesion of imponderable bodies to ponderable bodies see the part of this work which treats of Imponderables.] Respecting the adhesion of ponderable bodies to one another the following cases must be distinguished:

1. Adhesion between elastic fluids.

Diffusion of Gases.-All gases, even when under existing circumstances they do not enter into chemical combination, yet diffuse themselves through one another and form a uniform mixture, though their specific gravities may be very different and they may be kept externally at perfect rest. If, for example, two bottles be connected by an upright glass tube 10 inches long and inch wide, the upper bottle being filled with hydrogen, nitrogen, binoxide of nitrogen, or common air, and the lower with the heavier gas carbonic acid, or the upper with hydrogen and the lower with common air, nitrogen, oxygen or binoxide of nitrogen, a portion of the heavier gas will after a few hours be found in the upper bottle, and after two or three days both bottles will contain the two gases in the same proportion (Dalton, Phil. Mag. 24, 8). The same result was obtained by Berthollet (Mém. d'Arcueil, 2, 463) with a tube 10 inches long and of an inch wide placed in a cellar where no change of temperature could take place to set the gases in motion. When hydrogen was the gas contained in the upper vessel the two gases were found to be uniformly mixed in 1-2 days; but when air, oxygen, or nitrogen, was contained in the upper vessel and carbonic acid in the lower, several weeks elapsed before the mixture became perfectly uniform.

If a cylinder filled with any gas and placed in a horizontal position be made to communicate with the external air by means of a knee-shaped tube in such a manner that the end of the tube is directed downwards when the gas is lighter and upwards when it is heavier than the air, the gas will gradually escape from the cylinder, its place being supplied by the air. According to Graham,

Of 100 volumes of gas there disappeared,

From this it appears that gases escape the more quickly the lighter they are; and their expansive power or diffusibility probably varies in the inverse ratio of the square roots of their specific gravities. Thus 47 measures of hydrogen escaped in two hours, and the same volume of carbonic acid in 10. Now this proportion of 1:5 is nearly that of the square root of 1 (spec. grav. of hydrogen) to the square root of 22 (spec. grav. of carbonic acid.) (Graham.)

If the cylinder contain a mixture of 2 gases, the more diffusible of the two will escape in greater proportion into the air, and the less diffusible in smaller proportion than if each gas were contained in the cylinder alone. Thus of 50 measures of hydrogen and 50 of olefiant gas there escape in 10 hours 47-7 measures of the former and 12.5 of the latter: similarly 47 measures of hydrogen and 20 of carbonic acid; though in these cases the opening of the knee-shaped tube is directed downwards: further in 4 hours there escape 26-8 vol. of light carburetted hydrogen and 12.5 of carbonic acid, also 22.8 of light carburetted hydrogen and 18.6 of olefiant gas. If two bottles be connected together by a tube placed in a vertical position, the lower bottle being 7 times as large as the upper and filled with carbonic acid gas, while the upper one is filled with a mixture of hydrogen and olefiant gas in equal volumes, the upper vessel will after 10 hours be found to contain, besides carbonic acid, a quantity of olefiant gas whose volume is 4 times as great as that of the hydrogen still remaining; the latter has therefore, in spite of its greater levity, diffused itself through the lower vessel with greater rapidity. (Graham, Qu. Jour. of Sc. 6, 74; also Schw. 57, 215).

In the same manner also vapours diffuse themselves through one another and through the more permanently elastic fluids.

When different elastic fluids have once diffused themselves uniformly through one another they never separate again according to their different specific gravities, for however long a time the mixture may be left at rest; this was shown long ago by Priestley.

These gaseous mixtures differ essentially from all other mixtures in the following respects: their heterogeneous constitution cannot be detected by the eye; they transmit light without the slightest disturbance; and they cannot be decomposed by mechanical means. It must be observed however that gases when devoid of colour cannot be distinguished from one another by the eye; they are invisible, and a glass vessel presents the same appearance whether it is exhausted of air or filled with a colourless gas. When therefore two gases have by their natural adhesion diffused themselves through each other with that extreme uniformity which their great mobility and lightness render possible, it is not to be expected that their heterogeneous nature should be detected by the eye; and even the colour which some gases possess is by this extremely intimate mixture so much divided that even the microscope cannot distinguish the coloured and colourless particles of gas. This minute division also causes the rays of light to be uniformly refracted in the gaseous mixture and to go straight through: lastly the same cause must prevent mechanical separation, unless we can find sieves fine enough to let the smaller particles pass through them and stop the rest. This peculiarity of gaseous mixtures has led several chemists to propose the following theories which however are likely to be soon forgotten-respecting their nature. 1. Berthollet, Murray, and others regard a gaseous mixture as an imperfect chemical combination. Gas-mixtures are however destitute of all the characters of a chemical combination excepting uniformity: (a.)