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the assurance that, in consequence of the density of the sea being only one-fifth of the mean density of the earth, and the earth itself increasing in density towards the centre, the stability of the equilibrium of the ocean never can be subverted by any physical cause. A general inundation arising from the mere instability of the ocean is therefore impossible. A variety of circumstances, however, tend to produce partial variations in the equilibrium of the seas, which is restored by means of currents. Winds and the periodical melting of the ice at the poles occasion temporary watercourses; but by far the most important causes are the centrifugal force induced by the velocity of the earth's rotation, and variations in the density of the sea.

The centrifugal force may be resolved into two forces-one perpendicular, and another tangent to the earth's surface (N. 161). The tangential force, though small, is sufficient to make the fluid particles within the polar circles tend towards the equator, and the tendency is much increased by the immense evaporation in the equatorial regions from the heat of the sun, which disturbs the equilibrium of the ocean. To this may also be added the superior density of the waters near the poles, from their low temperature. In consequence of the combination of all these circumstances, two great currents perpetually set from each pole towards the equator. But, as they come from latitudes where the rotatory motion of the surface of the earth is very much less than it is between the tropics, on account of their inertia, they do not immediately acquire the velocity with which the solid part of the earth's surface is revolving at the equatorial regions ; from whence it follows that, within twenty-five or thirty degrees on each side of the line, the ocean has a general motion from east to west, which is much increased by the action of the trade winds. Both in the Pacific and Atlantic currents of enormous magnitude are deflected by the continents and islands to the north and south from this mighty mass of rushing waters, which convey the warmth of the equator to temper the severity of the polar regions, while to maintain the equilibrium of the seas counter currents of cold water are poured from the polar oceans to mingle with the warm waters at the line, so that a perpetual circulation is maintained.

Icebergs are sometimes drifted as far as the Azores from the Polar seas, and from the south pole they have come even to the

Cape of Good Hope. But the ice which encircles the south pole extends to lower latitudes by 10° than that which surrounds the north. In consequence of the polar current Sir Edward Parry was obliged to give up his attempt to reach the north pole in the year 1827, because the fields of ice were drifting to the south faster than his party could travel over them to the north.

Kotzebue and Sir James Ross found a stratum of constant temperature in the ocean at a depth depending upon the latitude: at the equator it is at the depth of 7200 feet, from whence it gradually rises till it comes the surface in both hemispheres about the latitude of 56° 26', where the water has the same temperature at all depths; it then descends to 4500 feet below the surface about the 70th parallel both in the Arctic and Antarctic Seas. The temperature of that aqueous zone is about 390.5 of Fahrenheit.* It divides the surface of the ocean into five great zones of temperature, namely, a medial region, in which the highest mean temperature is 82° Fahr., two temperate zones each of 390.5 Fahr., and two polar basins at the freezing point of salt water.

* See the chapter on the Tides and Currents in the Physical Geography,' by the author, 4th edition.

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Molecular Forces — Permanency of the ultimate Particles of Matter —

Interstices Mossotti's Theory — Rankin's Theory of Molecular Vortices — Gases reduced to Liquids by Pressure — Gravitation of Particles

- Cohesion - Crystallization - Cleavage — Isomorphism ---Minuteness of the Particles – Height of Atmosphere — Chemical Affinity- Definite Proportions and Relative Weights of Atoms — Faraday's Discovery

with regard to Affinity - Capillary Attraction. THE oscillations of the atmosphere, and its action upon the rays of light coming from the heavenly bodies, connect the science of astronomy with the equilibrium and movements of fluids and the laws of molecular attraction. Hitherto that force has been under consideration which acts upon masses of matter at sensible distances; but now the effects of such forces are to be considered as act at inappreciable distances upon the ultimate molecules of material bodies.

All substances consist of an assemblage of material particles, or molecules, which are far too small to be visible by any means human ingenuity has yet been able to devise, and which are much beyond the limits of our perceptions. They neither can be created nor destroyed; bodies may be burned, but their particles are not consumed—they are merely liberated from one combination to enter into another, nor are their peculiar properties ever changed ; whatever combinations they may 'enter into, they are ever and invariably the same.

Since every known substance may be reduced in bulk by pressure, it follows that the particles of matter are not in actual contact, but are separated by interstices; and it is evident that the smaller the interstitial spaces the greater the density. These spaces appear to be filled with air in some cases, as may be inferred from certain semi-opaque minerals and other substances becoming transparent when plunged into water. Sometimes they may possibly contain some unknown and highly elastic fluid, such as Sir David Brewster has discovered in the minute cavities of various minerals, which occasionally causes them to explode


under the hands of the lapidary ; but as it is inconceivable that the particles of matter should act upon one another without some means of communication, it is presumed that the interstices of material substances contain a portion of the ethereal medium with which the regions of space are filled.

The various hypotheses that have been formed as to the nature and action of the forces which unite the particles of matter, have been successively given up as science advanced, and now nothing decisive has been attained, although Professor Mossotti, of Pisa, by a very able analysis, has endeavoured to prove the identity of the cohesive force with gravitation. As the particles of material bodies are not in actual contact, he supposes that each is surrounded by an atmosphere of the ethereal medium, which he conceives to be electricity ; moreover he assumes that the atoms of the medium repel one another, that the particles of matter also repel one another, but with less intensity, and that there is a mutual attraction between the particles of matter and the atoms of the medium, forces which are assumed to vary inversely as the square of the distance.

Hence, when the material molecules of a body are inappreciably near to one another, they mutually repel each other with a force which diminishes rapidly as the infinitely small distance between the material molecules augments, and at last vanishes. When the molecules are still farther apart, the force becomes attractive. At that particular point where the change takes place the forces of repulsion and attraction balance each other, so that the molecules of a body are neither disposed to approach nor recede, but remain in equilibrio. If we try to press them nearer, the repulsive force resists the attempt; and if we endeavour to break the body so as to tear the particles asunder, the attractive force predominates and keeps them together. This is what constitutes the cohesive force, or force of aggregation, by which the molecules of all substances are united. The limits of the distance at which the negative action becomes positive vary according to the temperature and nature of the molecules, and determine whether the body which they form be solid, liquid, or aëriform.

Beyond this neutral point the attractive force increases as the distance between the molecules augments till it attains a maximum; when the particles are more apart, it diminishes; and, as soon as they are separated by finite or sensible distances, it varies


directly as their mass and inversely as the square of the distance, which is precisely the law of universal gravitation.

Thus, on the hypothesis that the mutual repulsion between the electric atoms is a little more powerful than the mutual repulsion between the particles of matter, the ether and the matter attract each other with unequal intensities, which leaves an excess of attractive force constituting gravitation. As the gravitating force is in operation wherever there is matter, the ethereal electric medium must encompass all the bodies in the universe; and, as it is utterly incomprehensible that the celestial bodies should exert a reciprocal attraction through a void, the Professor concludes that the ethereal electrical medium fills all space.

It is true that this connexion between the molecular forces and gravitation depends upon hypothesis ; but in the greater number of physical investigations some hypothesis is requisite in the first instance to aid the imperfection of our senses ; and when the phenomena of nature accord with the assumption, we are justified in believing it to be a general law.

Mr. Rankin's theory of molecular vortices, or the molecular structure of matter, is independent of electricity. According to his hypothesis, each atom of matter consists of an inappreciably small nucleus, encompassed by an elastic ethereal atmosphere which is retained in its position by attractive forces directed towards the molecule, whilst the molecules attract each other in the direction of straight lines joining their centres. The nuclei may either be solid, or a high condensation of the atmospheres which surround each with decreasing density. When the attraction between the molecules is such that the elasticity of the atmospheres is insensible, the body is a perfect solid, the rigidity of -which bears a certain definite proportion to the elasticity of the volume. When the atmospheres are less condensed and the attraction of the molecules merely produces a cohesive force sufficient to balance the atomic elasticity of the atmosphere, the body is a perfect liquid ; and when the attraction of the molecules is very small compared with the elasticity of their ethereal atmospheres, the body is a perfect gas. These atmospheres are supposed to be portions of the ethereal medium which penetrates into the interstices of every substance, and their elasticity to be due to the heat generated by the centrifugal force or oscillations among their atoms, for motion is the cause of heat, the force

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