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 to the surface in both hemispheres about the latitude of 56° 26ʹ, 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 39°·5 of Fahrenheit.[[7]] 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 39°·5 Fahr., and two polar basins at the freezing point of salt water.
SECTION XIV.
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.