Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/341

CONSTITUTION OF BODIES 311 ether fluids, a small portion of which, when placed in a large vessel, does not at once expand so as to fill the vessel uniformly, but remains in a collected mass at the bottom, even when the pressure is removed. These fluids are called liquids. When a liquid is placed in a vessel so large that it only occupies a part of it, part of the liquid begins to evaporate, or in other words it passes into the state of a gas, and this process goes on either till the whole of the liquid is evapor ated, or till the density of the gaseous part of the substance has reached a certain limit. The liquid and the gaseous portions of the substance are then in equilibrium. If the volume of the vessel be now made smaller, part of the gas will be condensed as a liquid, and if it be made larger, part of the liquid will be evaporated as a gas. The processes of evaporation and condensation, by which the substance passes from the liquid to the gaseous, and from the gaseous to the liquid state, are discontinuous processes, that is to say, the properties of the substance are very different just before and just after the change has been effected. But this difference is less in all respects tho higher the temperature at which the change takes place, and Cagniard de la Tour in 1822 1 first showed that several substances, such as ether, alcohol, bisulphide of carbon, and water, when heated to a temperature sufficiently high, pass into a state which differs from the ordinary gaseous state as much as from the liquid state. Dr Andrews has since 2 made a complete investigation of the properties of carbonic acid both below and above the temperature at which the phenomena of condensation and evaporation cease to take place, and has thus explored as well as established the continuity of the liquid and gaseous states of matter. For carbonic acid at a temperature, say of C., and at the ordinary pressure of the atmosphere, is a gas. If the gas be compressed till the pressure rises to about 40 atmo spheres, condensation takes place, that is to say, the sub stance passes in successive portions from the gaseous to the liquid condition. If we examine the substance when part of it is condensed, we find that the liquid carbonic acid at the bottom of the vessel has all the properties of a liquid, and is separated by a distinct surface from the gaseous carbonic acid which occupies the upper part of the vessel. But we may transform gaseous carbonic acid at C. into liquid carbonic acid at C. without any abrupt change, by first raising the temperature of the gas above 30. 92 C. which is the critical temperature, then raising the pressure to about 80 atmospheres, and then cooling the substance, still at high pressure, to zero. During the whole of this process the substance remains perfectly homogeneous. There is no surface of separation between two forms of the substance, nor can any sudden change ba observed like that which takes place when the gas is condensed into a liquid at low temperatures ; but at the end of the process the substance is undoubtedly in the liquid state, for if we now diminish the pressure to some what less than 40 atmospheres the substance will exhibit the ordinary distinction between the liquid and the gaseous state, that is to say, part of it will evaporate, leaving the rest at the bottom of the vessel, with a distinct surface of separation between the gaseous and the liquid parts. The passage of a substance between the liquid and the solid state takes place with various degrees of abruptness. Some substances, such as some of the more crystalline metals, seem to pass from a completely fluid to a completely Bolid state very suddenly. In some cases the melted matter 1 Annales de Chimie, 2e serie, xxi et xxii. i Phil. Trans. 1869, p. 575. appears to become thicker before it solidifies, but this may arise from a multitude of solid crystals being formed in the still liquid mass, so that the consistency of the mass becomes like that of a mixture of sand and water, till the melted matter in which the crystals are swimming becomes all solid. There are other substances, most of them colloidal, such that when the melted substance cools it becomes more and more viscous, passing into the solid state with hardly any discontinuity. This is the case with pitch. The theory of the consistency of solid bodies will be discussed in the article ELASTICITY, but the manner in which a solid behaves when acted on by stress furnishes us with a system of names of different degrees and kinds of solidity, A fluid, as we have seen, can support a stress only when it is uniform in all directions, that is to say, when it is of the nature of a hydrostatic pressure. There are a great many substances which so far corre spond to this definition of a fluid that they cannot remain in permanent equilibrium if the stress within them is not uniform in all directions. In all existiitg fluids, however, when their motion is such that the shape of any small portion is continually changing-, the internal stress is not uniform in all directions, but is of such a kind as to tend to check the relative motion of the parts of the fluid. This capacity of having inequality of stress called into play by inequality of motion is called viscosity. All real fluids are viscous, from treacle and tar to water and ether and air and hydrogen. When the viscosity is very small the fluid is said to be mobile, like water and ether. When the viscosity is so great that a considerable inequality of stress, though it produces a continuously increasing displacement, produces it so slowly that we can hardly see it, we are often inclined to call the substance a solid, and even a hard solid. Thus the viscosity of cold pitch or of asphalt is so great that the substance will break rather than yield to any sudden blow, and yet if it is left for a sufficient time it will be found unable to remain in equilibrium under the slight inequality of stress produced by its own weight, but will flow like a fluid till its surface becomes level. If, therefore, we define a fluid as a substance which cannot remain in permanent equilibrium under a stress not equal in all directions, we must call these substances fluids, though they are so viscous that we can walk on them without leaving any footprints. If a body, after having its form altered by the applica tion of stress, tends to recover its original form when the stress is removed, the body is said to be elastic. The ratio of the numerical value of the stress to tho numerical value of the strain produced by it is called the coefficient of elasticity, and the ratio of the strain to the stress is called the coefficient of pliability. There are as many kinds of these coefficients as there are kinds of stress and of strains or components of strains produced by them. If, then, the values of the coefficients of elasticity were to increase without limit, the body would approximate to the condition of a rigid body. We may form an elastic body of great pliability by dissolving gelatine or isinglass in hot water and allowing the solution to cool into a jelly. By diminishing the proportion of gelatine the coefficient of elasticity of the jelly may be diminished, so that a very small force is required to produce a large change of form in the substance. When the deformation of an elastic body is pushed

beyond certain limits depending on the nature of the sub-