215. Radiactividad del radio.
A pesar de la enorme diferencia en sus actividades relativas, la radiactividad del radio presenta muchas analogías estrechas con la del torio y el actinio. Ambas sustancias dan lugar a emanaciones que a su vez producen una «actividad inducida» (excited activity) en los cuerpos situados en su vecindad.
El radio, sin embargo, no da lugar a ningún producto intermedio entre el elemento mismo y la emanación que produce; o, en otras palabras, no existe en el radio ningún producto que corresponda al Th X en el torio.
Giesel first drew attention to the fact that a radium compound gradually increased in activity after preparation, and only reached a constant value after a month’s interval. If a radium compound is dissolved in water and boiled for some time, or a current of air drawn through the solution, on evaporation it is found that the activity has been diminished. The same result is observed if a solid radium compound is heated in the open air. This loss of activity is due to the removal of the emanation by the process of solution or heating. Consider the case of a radium compound which has been kept for some time in solution in a shallow vessel, exposed to the open air, and then evaporated to dryness. The emanation which, in the state of solution, was removed as fast as it was formed, is now occluded, and, together with the active deposit which it produces, adds its radiations to that