Dissociation, in chemistry, a term applied to the influence of heat and pressure on chemical action. The word was first employed by Henri Sainte-Claire Deville, who in November, 1857, read before the French academy of sciences a paper " On the Dissociation or Spontaneous Decomposition of Bodies under the Influence of Heat." He says: "By selecting a proper compound and heating it sufficiently, the distance between the molecules can be increased to such an extent that they will separate into their elementary condition. This is a spontaneous decomposition, not determined by any chemical action. I propose to call it the dissociation of compound bodies." In 1846 Grove showed that fused platinum could determine the decomposition of water into its elements. Deville repeated this experiment on a large scale by pouring fused platinum into water, He obtained an explosive mixture of hydrogen and oxygen, and believes that at the temperature of the fusion of platinum water is dissociated into its constituents. Analo-gous experiments can be performed on solids.

Debray showed that when Iceland spar is heated in a tube from which the air has been exhausted, no decomposition takes place in mercury vapor at 350° C, and a scarcely perceptible decomposition in sulphur vapor at 440° 0.; but at 860° C. in vapor of cadmium it becomes very perceptible, and goes on till the tension of the liberated carbon dioxide becomes equivalent to 85 millimetres of mercury; on raising the temperature to 1042° C. in vapor of zinc, more carbon dioxide is evolved. If, on the other hand, the apparatus be allowed to cool, the carbon dioxide is gradually reabsorbed by the quicklime, and a vacuum is reestablished in the apparatus. Lamy has applied these results to the construction of a pyrometer for the measurement of high temperatures. The apparatus consists of a porcelain tube glazed on both sides, filled with pure carbonate of lime; one end of the tube is closed, the other connected with a manometer. By reading the volume of gas in the pressure gauge, and consulting the tables of tension, the temperature is determined. According to Deville, there is a tension of dissociation analogous to the tension of vapors, and the evaporation of a liquid or the decomposition of a carbonate is subject to the same laws.

In Fownes's "Chemistry," the exceptions to the law of molecular vapor occupying twice the volume of hydrogen is explained on the principle of the dissociation of the vapors at the high temperature required for the determination of their vapor density. Several writers on geology, among them Fournet and T. Sterry Hunt, have had recourse to Deville's theory of dissociation to explain the origin of rocks and the action of forces in primeval chemistry. The force of chemical affinity appears to be suspended by great heat, so that at a high temperature, like that of the sun, we may imagine that chemical elements, such as oxygen, hydrogen, chlorine, and sodium, can exist in the gaseous state, intimately mixed, but chemically uncombined. Many of the phenomena attributed by Berzelius to catalytic action are now explained on the principle of dissociation. Since attention was called to the subject by Deville, a large number of bodies have been investigated with reference to the tension of dissociation, and the doctrine has been pushed to the determination of the temperature of combustion, also to the better understanding of efflorescence and the phenomena of vaporization.

The dissociation of carbonic acid was accomplished by Deville by heat; it has since been performed by Thenard by the electric current. Carbonic oxide, sulphurous acid, hydrochloric acid, ammonia, and hydriodic acid have been dissociated by various chemists. It is the opinion of Dumas that Deville's theory of the tension of dissociation is as important to chemistry as Dalton's law of the tension of vapors was to physics.