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1 to 7 planes of cleavaye (Blätterdurchgänge) 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 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 i 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 laminæ 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 laminæ 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 them selves 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,

In 4 hours.

In 10 hours. 1 Hydrogen

81.6

94.5 Light carburetted hydrogen

62.7 8.5 Ammonia

41:4

59.6

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Sp. gr.

8

43:4

14

Olefiant gas

34.9 22 Carbonic acid 32 Sulphurous acid 35.4 Chlorine

23.7

31.6
27.6

48.3 47.0 46.0 39.5

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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 io. 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 bowever 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.)

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It is not true that overy substance can combine with every other : e. g. water mixes with alcohol but not with oil ; on the contrary, every gas whether simple or compound mixes with every other; and the rapidity with which the mixture takes place depends not on the chemical nature of the gases, but only on their specific gravities. (6.) Agreeably to this theory Berthollet regards the evaporation of water and other bodies in the air below their boiling points as a chemical solution. But the quantity of any salt which water can dissolve increases with the quantity of water present; on the contrary, the quantity of water which evaporates in a given space is the same, whether the space be void of air or filled either with rarefied or condensed air : and the evaporation goes on most slowly in the last mentioned case, precisely that in which the quantity of the so-called solvent, the air, is the greatest. (c.) The mixing of gases is never attended with alteration of temperature, a phenomenon which always accompanies real chemical combinations. (d.) In most chemical combinations an alteration of volume takes place, but not in the mixing of gases. (e.) The refractive power of a gaseous mixture is, according to Biot and Arago, exactly the mean between the refractive powers of the individual gases which compose it ; such however is not the case with real chemical combinations of gases, e. g. of hydrogen and nitrogen producing ammonia. (f.) Change of colour is often observed in chemical combinations, never in the mixing of gases. (g.) When one body is combined with another, a third body may withdraw the first and unite it to itself, the combination taking place less easily in most cases but more easily in some than if the first body were in the free state. Thus sulphur takes oxygen from protoxide of nitrogen at a higher, but from nitric acid at a lower temperature than from oxygen gas; but this substance takes fire in a mixture of oxygen and nitrogen (atmospheric air) at exactly the same temperature as in pure oxygen gas. Alkaline sulphites abstract oxygen from the air as easily as from pure oxygen gas, but not from protoxide of nitrogen which is a true chemical compound of oxygen and nitrogen. That phosphorus enters into slow combustion in the air at a lower temperature than in oxygen gas is accounted for by the more rarefied condition of the oxygen in the air.

2. Dalton supposed that in elastic fluids every atom of ponderable matter is surrounded with a sphere of heat: an elastic fluid is therefore to be regarded as a collection of spheres of heat each having a ponderable atom in its centre. His first hypothesis was that the calorific spheres belonging to the same elastic fluid repel each other, but not those of any other elastic fluid, so that as far as any other such Auid is concerned they may be regarded as not existing. Hence, neglecting the atoms of ponderablo matter, which moreover are supposed to occupy an exceedingly small spaco, an elastic fluid may be regarded as a vacuum with respect to other elastic fluids: hence it is that bodies of this nature diffuse themselves rapidly through one another. To this it may be replied : (a) that experience shows that this diffusion when it takes place through narrow connecting tubes occupies several days, whereas according to the hypothesis it ought to be instantaneous; (b) that this supposed expansion of the gasos ought to be attended with reduction of temperature ; (c) that according to this hypothesis the atoms of the gases must often be brought into immediato contact, and consequently must combine when they have any affinity for each other : such however is not the case with the greater number of elastic fluids.

Dalton thereforo afterwards assumed that the spherules of different

1.

re

es

gases and vapours universally repel each other. He explained their mixture by supposing their spherules to be of different sizes, so that when they come in contact the different spherules press upon each other unequally and produce currents till the whole has become uniformly mixed. If however we calculate the size of the spherules on this hypothesis, but according to more accurate experiments, we find that all elastic fluids may be divided into 7 classes according to the magnitude of their gaseous spherules: if the volume of the gaseous spherules of sulphur-vapour be assumed equal to 1, that of oxygen gas, olefiant gas, phosphorus vapour, &c., will be 3, of hydrogen, nitrogen, and chlorine gases, &c., 6, that of hydrochloric acid gas, ammoniacal gas, &c., 12, and that of some others 9, 18, and 24. According to this hypothesis gases which belong to one and the same class and therefore have spherules of equal magnitude ought not to mix. Comp. Draper (Phil. Mag. J. 13, 241).

Mixture of gases likewise takes place when they are separated by the interposition of a porous body. In this more complicated case we have to consider, besides the various diffusibilities of the gases: (1.) The smallness of the pores which may favour the penetration of one gas rather than of the other. (2.) The different degrees of adhesion which the diaphragm exerts npon the different gases, by virtue of which the gas which adheres most powerfully penetrates the diaphragm most easily, and attaining the opposite surface, mixes with the other. (3.) The different degrees of affinity of the diaphragm for the gases (e. g. of water in a wet bladder) by which the gases are absorbed and carried to the other side of the diaphragm. The following cases require particular notice:

Cracks in Glass. Hydrogen gas, kept in a cracked receiver standing over water, escapes by degrees through the crack into the surrounding air, the water under the receiver rising to the height of 2; inch above the outer level. The remaining hydrogen contains 7 per cent. of nitrogen but no oxygen. If the receiver be filled with oxygen and nitrogen instead of hydrogen, nothing will escape from it.

In the same manner hydrogen escapes out of bottles closed even with well ground stoppers, if the stoppers are not greased. (Döbereiner, Veber neu entdeckte höchst merkwürdige Eigenschaften des Platins, Jena, 1825, s. 15.) The crack must be neither too narrow, in which case it would allow no hydrogen to pass, nor too wide, in which it would give passage to other gases as well; it often becomes too wide during the experiment from unequal pressure of the air. If the cracked receiver containing the hydrogen be placed over a trough of mercury and covered with an uncracked receiver containing air or carbonic acid, the mercury will rise in the inner receiver to the height of an inch or two, and sink in the same proportion in the outer; but when the difference of level amounts to about 2 inches, the air begins to enter through the crack. If the inner cracked receiver be filled with air and the outer with hydrogen, the mercury will rise in the outer and sink in the inner, proving that the hydrogen 'makes its way through the crack in opposition to its smaller specific gravity. (Magnus, Pogg. 10, 153.) Supposing it to be established that the remaining hydrogen is

free from all traces of air or carbonic acid, which however is questioned by Graham who supposes that an interchange takes place, then these experiments would show that the smaller atoms of hydrogen are capable of making their way through intervals which are impervious to the larger ptoms of carbonic acid, &c.

Bottles filled over mercury with detonating gas (a mixture of 1 vol. of

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