Dissociation is a term applied to the gradual decomposition of a substance by heat into simpler components, generally two; the decomposition ceasing on stopping the supply of heat, and recombination gradually occurring as the temperature falls. The amount of decomposition thus increases with the temperature until it reaches a maximum, when the decomposition is complete. The rate of dissociation at first increases, reaches a maximum amount, and then decreases. Since the discovery of the phenomenon by St. Claire Deville, who gave to it the present name, the investigation of dissociation changes has been of great importance in physical and general chemistry. Thus the abnormal vapour densities of many substances - e.g. phosphorus pentachloride (PCl5), ammonium chloride (NH4Cl), acetic acid, etc. - are satisfactorily explained in view of a partial or complete dissociation. For example, the compound NH4Cl at temperatures at which it is vaporised, is dissociated into NH3 and HCl (ammonia and hydrochloric acid gases), which recombine on cooling. As a result, the vapour-density found is only one-half that indicated by the formula. To obtain proofs that dissociation does occur, and to separate the products in such cases, various methods have been employed, and in many cases, as in the example given, the unequal rates of diffusion of the components [Diffusion] may be successfully applied.