A theory of matter based on the mathematical description of the relationship between the volumes, temperatures, and pressures of gases (P-V-T phenomena). This relationship is summarized in the so-called gas laws as follows: (1) Boyle's law (at constant temperature the volume of a gas is inversely proportional to its pressure); (2) Charles's law (at constant volume the pressure exerted by a gas is proportional to its absolute temperature); (3) Avogadro's law (equal volumes of the same or different gases under the same conditions of temperature and pressure contain the same number of molecules).The theory involves the basic concept of matter as composed atoms and/or molecules that move more rapidly (gases) or vibrate more energetically (solids) as temperature increaes. Thus, crystals melt at a point where the heat or energy input exceeds the bond energy of the solid state.See Kinetics, Chemical; Gas; Thermodynamics.