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producing the motions varying simply as the density of the ether.

In aëriform fluids, although the particles are more remote from each other than in liquids and solids, yet the pressure may be so great as to reduce an aëriform fluid to a liquid, and a liquid to a solid. Dr. Faraday has reduced some of the gases to a liquid state by very great compression; but although atmospheric air is capable of a diminution of volume to which we do not know a limit, it has hitherto always retained its gaseous qualities, and resumes its primitive volume the instant the pressure is removed. Substances are said to be more or less elastic, according to the facility with which they regain their bulk or volume when the pressure is removed ; thus liquids resist compression on account of their elasticity, and in solids the resistance is much greater but variable, and the effort required to break a substance is a measure of the cohesive force exerted by its particles. In stone, iron, steel, and all brittle and hard substances, the cohesion of the particles is powerful but of small extent; in elastic bodies, on the contrary, its action is weak, but more extensive. An infinite variety of conditions may be observed in the fusion of metals and other substances passing from hardness to toughness, viscidity, and through all the other stages to perfect fluidity and even to vapour. Since all bodies expand by heat, the cohesive force is weakened by increase of temperature. The cohesion of matter or the strength of substances forms an important branch of study in engineering.

Every particle of matter, whether it forms a constituent part of a solid, liquid, or aëriform fluid, is subject to the law of gravitation. The weight of the atmosphere, of gases and vapour, shows that they consist of gravitating particles. In liquids the cohesive force is not sufficiently powerful to resist the action of gravitation. Therefore, although their component particles still maintain their connexion, the liquid is scattered by their weight, unless when it is confined in a vessel or has already descended to the lowest point possible, and assumed a level surface from the mobility of its particles and the influence of the gravitating forces, as in the ocean, or a lake. Solids would also fall to pieces by the weight of their particles, if the force of cohesion were not powerful enough to resist the efforts of gravitation.

The phenomena arising from the force of cohesion are innu

merable. The spherical form of rain-drops ; the difficulty of detaching a plate of glass from the surface of water; the force with which two plane surfaces adhere when pressed together ; the drops that cling to the window-glass in a shower of rain are all effects of cohesion entirely independent of atmospheric pressure, and are included in the same analytical formula (N. 162) which expresses all the circumstances accurately, although the laws according to which the forces of cohesion and repulsion vary are unknown. It is more than probable that the spherical form of the sun and planets is due to the force of cohesion, as they have every appearance of having been at one period in a state of fusion.

A very remarkable instance has occasionally been observed in plate-glass manufactories. After the large plates of glass of which mirrors are to be made have received their last polish, they are carefully wiped and laid on their edges with their surfaces resting on one another. In the course of time the cohesion has sometimes been so powerful, that they could not be separated without breaking. Instances have occurred where two or three have been so perfectly united, that they have been cut and their edges polished as if they had been fused together; and so great was the force required to make the surfaces slide that one tore off a portion of the surface of the other.

In liquids and gases the forms of the particles have no influence, they are so far apart; but the structure of solids varies according to the sides which the particles present to one another during their aggregation. Nothing is known of their form further than the dissimilarity of their different sides in certain cases, which appears from their reciprocal attractions during crystallisation being more or less powerful according to the sides they present to one another. Crystallisation is an effect of molecular attraction regulated by certain laws, according to which atoms of the same kind of matter unite in regular forms— a fact easily proved by dissolving a piece of alum in pure water. The mutual attraction of the particles is destroyed by the water; but, if it be evaporated, they unite, and form in uniting eightsided figures called octahedrons (N. 163). These however are not all the same. Some have their angles cut off, others their edges, and some both, while the remainder take the regular form. It is quite clear that the same circumstances which cause the

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aggregation of a few particles would, if continued, cause the addition of more; and the process would go on as long as any particles remain free round the primitive nucleus, which would increase in size, but would remain unchanged in form, the figure of the particles being such as to maintain the regularity and smoothness of the surfaces of the solid and their mutual inclinations. A broken crystal will by degrees resume its regular figure when put back again into the solution of alum, which shows that the internal and external particles are similar, and have a similar attraction for the particles held in solution. The original conditions of aggregation which make the molecules of the same substance unite in different forms must be very numerous, since of carbonate of lime alone there are many hundred varieties; and certain it is, from the motion of polarised light through rock crystal, that a very different arrangement of particles is requisite to produce an extremely small change in external form. A variety of substances in crystallising combino chemically with a certain portion of water which in a dry state forms an essential part of their crystals, and, according to the experiments of MM. Haidinger and Mitscherlich, seems in some cases to give the peculiar determination to their constituent molecules. These gentlemen have observed that the same substance crystallising at different temperatures unites with different quan, tities of water and assumes a corresponding variety of forms, Seleniate of zinc, for example, unites with three different portions of water, and assumes three different forms, according as its temperature in the act of crystallising is hot, lukewarm, or cold. Sulphate of soda also, which crystallises at 90° of Fahrenheit without water of crystallisation, combines with water at the ordinary temperature, and takes a different form. Heat appears to have a great influence on the phenomena of crystallisation, not only when the particles of matter are free, but even when firmly united, for it dissolves their union, and gives them another determination. Professor Mitscherlich found that prismatic crystals of sulphate of nickel (N. 164), exposed to a summer's sun in a close vessel, had their internal structure so completely altered without any exterior change, that when broken open they were composed internally of octahedrons with square bases. The original aggregation of the internal particles had been dissolved, and a disposition given to arrange themselves in a crystalling


form. Crystals of sulphate of magnesia and of sulphate of zinc, gradually heated in alcohol till it boils, lose their transparency. by degrees, and when opened are found to consist of innumerable minute crystals totally different in form from the whole crystals ; and prismatic crystals of zinc (N. 165) are changed in a few seconds into octahedrons by the heat of the sun : other instances might be given of the influence of even moderate degrees of temperature on molecular attraction in the interior of substances. It must be observed that these experiments give entirely new views with regard to the constitution of solid bodies. We are led from the mobility of fluids to expect great changes in the relative positions of their molecules, which must be in perpetual motion even in the stillest water or calmest air; but we were not prepared to find motion to such an extent in the interior of solids, That their particles are brought nearer by cold and pressure, or removed farther from one another by heat, might be expected; but it could not have been anticipated that their relative positions could be so entirely changed as to alter their mode of aggregation. It follows, from the low temperature at which these changes are effected, that there is probably no portion of inorganic matter that is not in a state of relative motion.

Professor Mitscherlich's discoveries with regard to the forms of crystallised substances, as connected with their chemical character, have thrown additional light on the constitution of material bodies. There is a certain set of crystalline forms which are not susceptible of variation, as the die or cube (N. 166), which may be small or large, but is invariably a solid bounded by six square surfaces or planes. Such also is the tetrahedron (N. 167) or four-sided solid contained by four equal-sided triangles. Several other solids belong to this class, which is called the Tessular system of crystallisation. There are other crystals which, though bounded by the same number of sides, and having the same form, are yet susceptible of variation ; for instance, the eight-sided figure with a square base, called an octahedron (N. 168), which is sometimes flat and low, and sometimes acute and high. It was formerly believed that identity of form in all crystals not belonging to the Tessular system indicated identity of chemical composition. Professor Mitscherlich, however, has shown that substances differing to a certain degree in chemical composition have the property of assuming the same

crystalline form. For example, the neutral phosphate of soda and the arseniate of soda crystallise in the very same form, contain the same quantities of acid, alkali, and water of crystallisation ; yet they differ so far, that one contains arsenic and the other an equivalent quantity of phosphorus. Substances having such properties are said to be isomorphous, that is, equal in form. Of these there are many groups, each group having the same form, and similarity though not identity of chemical composition. For instance, one of the isomorphous groups is that consisting of certain chemical substances called the protoxides of iron, copper, zinc, nickel, and manganese, all of which are identical in form and contain the same quantity of oxygen, but differ in the respective metals they contain, which are, however, nearly in the same proportion in each. All these circumstances tend to prove that substances having the same crystalline form must consist of ultimate atoms having the same figure and arranged in the very same order; so that the form of crystals is dependent on their atomic constitution.

All crystallised bodies have joints called cleavages, at which they split more easily than in other directions; on this property the whole art of cutting diamonds depends. Each substance splits in a manner and in forms peculiar to itself. For example, all the hundreds of forms of carbonate of lime split into six-sided figures, called rhombohedrons (N. 169), whose alternate angles measure 105.55° and 75.05°, however far the division may be carried ; therefore the ultimate particle of carbonate of lime is presumed to have that form. However this may be, it is certain that all the various crystals of that mineral may be formed by building up six-sided solids of the form described, in the same manner as children build houses with miniature bricks. It may be imagined that a wide difference may exist between the particles of an unformed mass and a crystal of the same substance between the common shapeless limestone and the pure and limpid crystal of Iceland spar; yet chemical analysis detects none; their ultimate atoms are identical, and crystallisation shows that the difference arises only from the mode of aggregation. Besides, all substances either crystallise naturally, or may be made to do so by art. Liquids crystallise in freezing, vapours by sublimation (N. 170); and hard bodies, when fused, crystallise in cooling. Hence it may be inferred that all substances are

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