Compression and Compressibility. When a body is subjected to the action of any force which causes it to occupy less volume, it is said to be compressed, and the diminution of volume is termed compression. The term compressibility is frequently used to signify that property of bodies whereby they yield to that particular form of stress known as pressure; but more strictly it is employed to denote the measure of this property as possessed by different substances. Under the same pressure it is obvious that the same volume of various substances will diminish by different amounts; and, to measure this change, the compressibility is defined to be the ratio of the amount of compression per unit volume to the compressing force applied. It thus may be determined by measuring the amount of compression of a known volume when under a certain pressure; dividing this by the product of the original volume and the pressure gives the average compressibility (per unit pressure) of the substance throughout the range of pressure employed. The unit of pressure generally used is one atmosphere, which is defined in this country as being the weight of a column of mercury, one square inch in section, 29.905 inches in height, at the temperature of 0° C., and weighed at sea-level in the latitude of London. Its actual value in pounds-weight per square inch is nearly 14.7; so that 152.3 atmospheres of pressure is equivalent to a pressure of one ton per square inch.
In gases the relation between pressure and volume is given by Boyle's Law (see GASES)—viz. the volume of a given mass of gas is inversely proportional to its pressure. From this it follows that the compressibility is inversely proportional to the pressure—i.e. the diminution of volume due to a given increment of pressure is correspondingly small as the pressure is great. The behaviour of a gas under pressure is closely related to the proximity of its temperature to the critical point (see CRITICAL TEMPERATURE); for if below this temperature the gas can, and if above it, cannot be liquefied by pressure alone. It is only since 1877 that liquefaction has been effected in those gases formerly termed permanent.
From the first attempts to compress liquids it was concluded that they were incompressible, but Canton in 1762, by a comparatively simple experiment, showed that the compressibility of water though small is quite appreciable, and that it is less at higher than at lower temperatures. The measurement of the compressibility of liquids is usually made in a glass vessel (see fig.) termed a piezometer. A tube, ABCD, open at one end, D, is bent upon itself between C and D, widened at one end into a cylindrical bulb, AB, and at the other into a cistern, D. The liquid experimented on fills the bulb and stem to C, from which point to D, mercury fills the tube. On the surface of the mercury at C an index floats. The instrument is placed in a larger and much stronger vessel containing water to which pressure (measured by an attached gauge) is applied. The contents of the piezometer being thus compressed, the mercury column ascends in the stem, and when the pressure is relieved the index is left at that point to which the mercury rose under the highest pressure applied. The actual amount of compression, and the original volume, as well as the pressure, being known, the compressibility can be thereby calculated, a correction being finally added for the compression of the glass piezometer itself. From experiments made with such apparatus, the following conclusions (see Report on some of the Physical Properties of Fresh Water and Sea-water, by Professor P. G. Tait; Challenger Expedition Commission Reports, Physics and Chemistry, part iv.) seem now to be well established regarding the compressibility of liquids, more especially of water. The compressibility of water decreases as both the temperature and pressure are raised; under moderate pressures (e.g. one or two atmospheres) it has a point of minimum value about 60° C., while its actual value at 10° C. and at a pressure of one ton per square inch is very nearly . Sea-water is less compressible than fresh water; the ratio of the compressibility of the former to the latter being .915. Solutions of common salt are less compressible as they are stronger; the compressibility falling off uniformly with increased strength. Both sea-water and salt solutions diminish in compressibility with temperature and pressure in the same manner as fresh water. It has also been proved that the maximum-density point of water is lowered by pressure; the actual amount of this lowering being 3°·1 C. per ton—i.e. water under a pressure of one ton per sq. in. has its maximum density point at 0°·9, instead of at 4°, as under ordinary atmospheric pressure.
The compressibility of solids is generally very much smaller than that of either liquids or gases. It is best measured by noting the shortening of a rod or fibre of the material tested while subjected to hydrostatic pressure; the linear compressibility thus obtained is, to a sufficient degree of approximation, one-third the cubical compressibility. For glass it is .00000265 per atmosphere.