M. Debierne has shown that actinium causes, to a marked degree, induced activity of bodies placed in its vicinity. As in the case of thorium, there is a considerable carriage of activity by air currents.
Induced radio-activity has various aspects, and irregular results are obtained when the activity of a substance in the neighbourhood of radium is excited in free air. MM. Curie and Debierne have observed, however, that the phenomenon is quite regular when operating in a closed vessel; they therefore investigated induced activity in a closed space.
Activity Induced in an Enclosed Space.
The active material is placed in a little glass jar, a, open at O (Fig. 11), in the centre of a closed space. Several plates, A, B, C, D, E, placed in the enclosure become radio-active after one day’s exposure. The activity is the same whatever be the nature of the plate, for equal dimensions (lead, copper, aluminium, glass, ebonite, wax, cardboard, paraffin). The activity of one face of one of the plates is greater in proportion to the amount of free space about this face.
If the preceding experiment be repeated with the jar, a, completely closed, no activity is induced.
The radiation of radium does not directly affect the production of induced radio-activity. For this reason, in the preceding experiment the plate D, screened from the radiation by a lead plate of thickness, P P, is made as active as B and E.
Radio-activity is transmitted by the air by degrees from the radiating body to the body to be excited. It can even be transmitted to a distance by very narrow capillary tubes.
Induced radio-activity is both more intense and more regular if the solid radium salt be replaced by an aqueous solution of the same.
Liquids are capable of acquiring induced radio-activity. For example, pure water may be rendered active by placing it with a solution of a radium salt within an enclosure.
