1. In soil science, the process by which radioactive gases escape from the surface layers of soil or loose rock where they are formed by decay of radioactive salts. The exhalation of radioactive gases, notably radon and thoron, increases with soil temperature and so normally exhibits a single daily maximum around midday. Decreases of atmospheric pressure normally increase the exhalation, and freezing of the surface soil layers usually greatly reduces it. 2. The streaming forth of volcanic gases; also the escape of gases from a magnetic field.