1. In atmospheric electricity, any one of the three radioactive inert gases, radon, thoron, and actinon, that contributes to atmospheric ionization by virtue of the ionizing effect of the alpha particles that each emits on disintegration. These three gases are isotopic to each other, all having atomic number 86. They are members of distinct families of radioactive elements, but each is formed as a result of alpha emission and each decays by that process. They form in the interstices of soil or porous rocks containing their respective parent atoms in the forms of salts or minerals. By the process of exhalation, they enter the surface layers of the atmosphere and are then carried upward by turbulence and convection. 2. Any gaseous material containing radioactive atoms.