Compressibility, that property of matter which allows the volume of a body to be diminished by pressure, being a consequence as well as an evidence of porosity. The word porosity as used here does not signify the ordinary sensible porosity of bodies, such as is observed in a sponge or in wood, but that which has received the name of physical porosity. The physical pores are the exceedingly small interstices between the molecules of matter of which a body is formed, so minute in solids and liquids as to allow the molecules to remain within the sphere of cohesive attraction. Examples of compressibility are furnished by the reduction of liquids under pressure, and of compact masses of metal by hammering. The relative compressibilities of different bodies are readily ascertained by subjecting them to a given pressure and observing the diminution of volume resulting in each, care being taken that the temperatures are the same, because each degree of heat increases the repulsion between the molecules. Under the same pressure, a solid heated is larger than when cold. When bodies are compressed sensible heat is always manifested, the force being converted to this condition.

Gases and vapors are 'the most compressible of all bodies, and within certain limits their compressibility is uniform and in proportion to the compressing force, as enunciated in Mariotte's law. The forcing of water through the sides of a heavy hollow ball of gold in which the liquid was confined, in the attempt of the Florentine academicians to compress it, led to the long continued belief in the total incompressi-bility of liquids; but this property has been proved by Canton, Oersted, and others to exist in them as well as in gases and solids. Later experiments by Colladon and Sturm show that an additional pressure equal to that of the atmosphere caused mercury to diminish 1/5,000,000 of its original volume, water 1/500,000, and ether 133/1,000,000.