COMPRESSIBILITY is that property of bodies by which they admit of being forced or pressed into less space than they formerly occupied. The particles composing bodies are in all cases at greater or less distances from one another; and whatever brings the 1)articles closer together, diminishes the volume or bulk of the body. This may be effected by various agencies, as, e.g., by the withdrawal of heat (q.v.); but the effect is called compression only when it is caused by mechanical force, as by pressure or per cussion. All bodies are compressible, but in different degrees. Many solids, especially those of a compact structure, have this property only in a slight degree. It was believed at one time that liquids were incompressible; more accurate experiments, however, have proved that this is not the case; water, for instance, subjected to a pressure of 15,000 lbs. on the sq.in., loses of its volume. Gases, on the other hand, are strikingly coin by means of a common condensing syringe, a number of cubic inches of air can be forced into the space of one inch. Compression is in almost every instance
accompanied by an evolution of heat. When a piston, having a piece of German tinder attached to the bottom, is forced rapidly to the bottom of a shut condensing syringe, and rapidly withdrawn, the tinder is found ignited.
In a restricted sense, those gases are said to be compressible which, under great pres sure, become liquid. This is the case with carbonic-acid gas, chlorine, sulphurous-acid and others. Atmospheric air and its components have hitherto resisted all attempts to liquefy them; though it is believed that only a sufficient degree of pressure and cold is necessary to make any gas liquid. Carbonic-acid and some other gases are liquefied in small quantities by inclosing the ingredients necessary for generating the gas in a strong glass tube, keeping them separate till the tube is hermetically sealed. The gas, as it is produced, is condensed into a fluid by its own pressure, which is aided by keeping one end of the bent tube in a cooling mixture.