CONSTITUTION OF MATTER. The ATOMIC constitution of bodies is discussed when we consider whether they consist of finite ultimate particles called atoms, or are infi nitely divisible. The CHENIICAL constitution of bodies is the mode in which elementary substances unite to form compounds. The PHYSICAL constitution of bodies is the mode in which particles of matter in forms cognizable by sense, are aggregated to form masses of known qualities. Bodies arc classed according to their ability to retain their dimen sions when they are subjected to internal stress. A solid retains its dimensions even under the influence of a stress which acts in but a single direction. A fluid changes it§ dimensions unless the stress acting within it is uniform in all directions. If the fluid is capable of expanding indefinitely to fill the vessel which contains it, it is a gas: if, under ordinary circumstances, the fluid does not so expand, but remains collected in some part of the vessel, it is a liquid. The lines of demarkation between these states of matter can not be sharply drawn; the conditions merge insensibly, the one into the other. Lead is classed as a solid, yet if submitted to suitable pressure, its particles move upon each other, and the given mass takes a new form, as when bullets arc made by the cold pro cess from rods of lead. Sulphuric ether is usually a liquid, which, under ordinary con ditions, occupies part of a vessel without expanding to fill the whole; but if pressure be removed, as in the receiver of an air-pump, the ether assumes the gaseous form, and resumes the form of a liquid when pressure is restored. Under suitable variations of volume and pressure, part of the liquid may be vaporized, or part of the vapor may be liquefied. Experiments indicate that all liquids may be vaporized, as it is known that all gases and vapors may be liquefied. Substances pass from a liquid to a solid state, or rice versil, with different degrees of abruptness. The change occurs suddenly in most metals, as gold, copper, lean, while iron and glass melt by degrees, and harden slowly; whence it comes that iron and glass can be welded, while gold, copper, etc., cannot. When the particles of water, iron, sulphur, etc., solidify, they arrange themselves accords ing to some law of symmetry, forming crystals; other substances, as pitch, becoming constantly more and more viscous, solidify by imperceptible degrees, and without sym metrical or crystalline arrangement of particles. This quality of viscosity indicates a certain attraction between the particles of a fluid, by which they have some power of resisting the stress to which they ultimately yield. In this respect a series of sub stances may be arranged, beginning with fluids of extreme limpidity, as water, alcohol, ether, passing through certain oils, molasses, tar, moltenglass, that may be drawn out into strings, to the metals which even when cold may be drawn into wire, or rolled into sheets. Prof. Forbes explains the motion of the glacier by the viscosity of ice. A substance may seem to lack this property when struck sharply, and may yet show it under constant, though moderate, pressure; thus pitch, when cold, breaks under the hammer, and slowly bends to fit the surface on which it rests.
A body is said to be elastic, if, after its dimensions have changed under the influence of a stress, it resumes its original dimensions when the stress is removed. The numerical ratio of stress to strain, or deformation, is called the of elasticity; that of strain to stress, the of pliability. Thus, if 10,000 lbs., applied to a wire of assumed unit of cross section, elongate the wire one per cent, the co-efficient of elasticity of the wire is = 1,000,000 lbs. per unit of section. If the stress upon an elastic body be pushed beyond certain limits, different for different substances, the body does not return to its former dimensions, but remains permanently deformed, or may be parted. The limit beyond which the body becomes permanently deformed is the limit of elasticity. The limit beyond which it breaks is called the limit of tenacity. The body is soft, if it may be deformed under a small force; it is tough, if a large force is required. If nip hire occurs before deformation, the body is brittle; if it resists great force with neither rupture nor deformation, it is hard. The stiffness of a body is the measure of its ability to resist deformation; its strength, the measure of the force required to break it.
The changes which occur in molecular deformation have been explained thus: We know that the molecules of all bodies are in motion. In fluids the motion is such that any molecule may pass freely from one part of the mass to another; in solids we may suppose that sonic, at least, of the particles merely oscillate about a certain position so that the configuration of a group of molecules is never much changed from a certain stable form about which they oscillate. If the amplitude of the oscillation exceeds a certain limit, the molecules, failing to return to their old relations, assume new figures of stability, in which the strains are less than in the former figures, and may be zero. This breaking up of configuration may result front wide amplitude of vibration, or from great strain, and we may suppose that the different groups of amass may have dif ferent powers of resistance in either of these respects. We may further suppose that as the groups break up, some of the resulting configurations are suali as correspond to a _ - strain which is uniform in all directions. In the ratio in which the groups reach this result, the mass approaches the condition of a fluid of greater or less viscosity. If the mass be conceived as made up of molecules which differ in stability of configuration, it is possible that the weaker have yielded to a force which the stronger have been able to resist, and that the latter retain their original condition; that, subsequently, other forces, as change of temperature or of moisture, or violent vibration, may cause a farther dis integration of the groups of low stability; that then the more stable groups have oppor tunity to control these weaker groups, and gradually restore the whole mass to the con dition and form which it had before any deformation occurred.