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# Compressibility

Compressibility. That property of matter by which it may be diminished in volume when subjected to pressure. All substances, solid, liquid, and gaseous, are to some extent compressible. Gases submit to diminution in volume readily, the intensity of pressure required to effect this being inversely proportional to the volume. Thus, if the gas be compressed to half its bulk, the new pressure which it exerts, and which must therefore be exerted on it, will be double the former pressure. This is known as Boyle's Law (q.v.). Liquids and solids, on the other hand, offer very great resistance to change of volume, though the former are changed in shape under the action of slight distorting forces. The famous Florentine experiment, in which a hollow sphere of gold, filled with water, allowed so little compression that the water oozed through the gold shell when subjected to great pressure, was regarded for some time as a proof of the non-compressibility of water. Nevertheless an instrument called the piezometer demonstrates and measures the diminution in volume of water or other liquids under pressure. Water diminishes 1/20000th of its volume under a pressure of one ton per square inch. It must be understood, however, that, under special circumstances, the liquid condition of matter may merge imperceptibly into the gaseous. Towards this critical condition of temperature and pressure it is found that the liquid suffers much change in volume for slight changes in pressure. The compressibility of solids, as a rule, is less even than that of liquids. The compression of ice at 0° C will convert it to water, and the compression of steam at boiling-point will convert it to water, for pressure lowers the freezing-point and raises the boiling point of that liquid.