GASES, GENERAL PROPERTIES OF. The study of the nature and properties of gases has yielded many of the most important results of modern science. Practically the entire structure of modern chemistry rests on our knowledge of gases. The birth of the science, as already ex plained in the article CHEMISTRY, followed al most immediately the discovery of the common gases. The fruitful theories of modern organic chemistry are based entirely on the general prop erties of gases; and in the latter part of the nineteenth century general theoretical chemistry received a powerful impulse by the extension of the laws of gases to dilute solutions. (See Soix TroN.) On the other hand, the . physicist has been led, by the study of gases, to a clear and simple explanation of the phenomena of heat and of many other general phenomena forming im portant chapters in modern physics. And, of course, through chemistry and physics the ap plied and natural sciences, too, owe a great deal to our knowledge of gases. All this importance of gases is due to the comparative simplicity of the laws followed by them. The simplicity of the laws is, in turn, readily explained from the standpoint of the molecular conception. Mole cules are minute particles of matter. When they are very near to one another there must natu rally come into play, between them, forces whose effects are practically nothing when the mole cules are widely separated. Under ordinary pressures a substance occupies a much greater volume in the gaseous than in the liquid or solid state. Thus, an amount of water occupying, at 0° C., one cubic centimeter if liquid, would, if vaporized at the same temperature and under ordinary atmospheric pressure, occupy over 773 cubic centimeters. Evidently the molecules of a gaseous substance must be very far apart, and their mutual influence very slight. In other words, the number of causes determining the properties of gases must be smaller, and hence the properties themselves must be less complex than those of liquids or solids. Of course, as the volume within which a gas is compressed is made smaller and smaller, the relative simplicity of properties gradually disappears. (See MOLE CULES—MOLECULAR WEIGHTS.) Under certain conditions of pressure and temperature, the prop erties of a substance in the gaseous and liquid states even become identical. (See' CRITICAL
POINT.) This shows that simplicity of prop erties, while generally found in the gaseous state, is not strictly characteristic of it. Other char acteristics may be found mentioned under AG GREGATION, STATES OF.
It is explained in the articles on HYDROSTATICS and HYDRODYNAMICS how liquids and gases have certain properties in common, viz. all those which depend upon fluid pressure, which is de fined as the force per unit area. It is shown in those articles: (1) The pressure at any point in a gas is the same in all directions, and its value is ggh P, where p is the average density of the gas above the point, g is the acceleration due to gravity of a freely falling body, h is the vertical distance from the point to the top of the gas (if it is inclosed in a reservoir), and P is a pressure uni form throughout the gas, due to the reaction of the walls of the reservoir against the outward expansive force of the gas. In all ordinary cases of gases h is not large, and so agh may be neg lected, because s is extremely small; and P is the principal term. In the case of the atmos phere, however, P is zero and h large.
(2) The pressure of the gas against the con taining walls or against any solid immersed in it is perpendicular to the solid, if the gas is not flowing.
(3) Archimedes's principle applies to gases, viz. if a solid or a drop of liquid is immersed in the gas it is buoyed up with a force equal to the weight of the displaced gas.
(4) If a gas escapes from a reservoir through a small opening in a thin wall, its velocity of `efflux' is given by the formula v where. p P is the difference in pressure of the gas inside the reservoir and outside. (This is not the total pressure, but the partial pressure due to this particular gas. See Dalton's below.) (5) If a gas is flowing steadily but slowly through a tube or irregular cross-section, the pressure is greatest where the velocity is least, and vice versa. This is the principle of the `atomizer,' the 'injector' for steam boilers, etc.
The densities of gases at 0° C. and standard pressure are as follows: Air, 0.001293; carbon dioxide, 0.01974; hydro gen, 0.0000996; oxygen, 0.001430. The special properties of gases have been stated in the form of laws: