Gas

GAS, a general term for one of the three states of aggregation of matter ; also more specifically applied to coal-gas, the gaseous product formed in the destructive distillation of coal or other carbonaceous matter (see below, section Gas Manufacture; for gas engines see INTERNAL COMBUSTION ENGINES).

The Gaseous State.

Matter is studied under three physical phases—solids, liquids and gases, the latter two being sometimes grouped as "fluids." The gaseous fluid with which we have chiefly to do is our atmosphere. Though practically invisible, it appeals in its properties to other of our senses ; thus we feel it in its motion as wind, and observe the dynamical effects of this motion in the quiver of the leaf or the motion of a sailing ship.

The practically obvious distinction between solids and fluids may be stated in dynamical language thus : Solids can sustain a longi tudinal pressure without being supported by a lateral pressure; fluids cannot. Hence any region of space enclosed by a rigid boundary can be easily filled with a fluid, which then takes the form of the bounding surface at every point of it. But here we distinguish between fluids according as they are gases or liquids. The gas will always completely fill the region, however small the quantity put in. Remove any portion and the remainder will expand so as to fill the whole space again. On the other hand, it requires a definite quantity of liquid to fill the region. To quote Sir Oliver Lodge : "A solid has volume and shape ; a liquid has volume, but no shape; a gas has neither volume nor shape." It is necessary to distinguish further between a gas and a "vapour." The latter possesses the physical property stated above which distinguishes a gas from a fluid, but it differs from a gas by being readily condensible to liquid, either by lowering the temper ature or moderately increasing the pressure. The study of the effects of pressure and temperature on many gases led to the intro duction of the term "permanent gases," to denote gases which were apparently not liquefiable ; but with improved methods these gases have been liquefied and even solidified, thus rendering the term meaningless. (See LIQUEFACTION OF GASES.) "Perfect gas" is applied to an imaginary substance in which there is no frictional retardation of molecular motion, i.e., one which obeys Boyle's law. (See PHYSICS.) The study of gases may be divided into two main branches : the physical and the chemical. The former investigates essen tially general properties, such as the weight and density, the relation between pressure, volume and temperature (piezometric and thermometric properties), calorimetric properties, diffusion, viscosity, electrical and thermal conductivity, etc., and generally properties independent of composition. These subjects are dis cussed in the articles DENSITY; THERMOMETRY; DIFFUSION ; HEAT, Conduction of ; and CONDENSATION OF GASES. The latter has for its province the preparation, collection and identification of gases, and the volume relations in which they corn bine ; in general it deals with specific properties. The historical development of the chemistry of gases—pneumatic chemistry—is treated in the article CHEMISTRY ; the technical analysis of gaseous mixtures is treated under CHEMISTRY, Gas Analysis. Connecting the experimental study of the physical and chemical properties is the immense theoretical edifice termed the kinetic theory of gases. This subject, which is discussed in the article MOLECULE, has for its purpose, (I) the derivation of a physical structure of a gas which will agree with the experimental observations of the diverse physical properties, and (2) a correlation of the physical properties and chemical composition.