=175. Excited radio-activity.= One of the most interesting and remarkable properties of thorium, radium, and actinium, is their power of “exciting” or “inducing” temporary activity on all bodies in their neighbourhood. A substance which has been exposed for some time in the presence of radium or thorium behaves as if its surface were covered with an invisible deposit of intensely radio-active material. The “excited” body emits radiations capable of affecting a photographic plate and of ionizing a gas. Unlike the radio-elements themselves, however, the activity of the body does not remain constant after it has been removed from the influence of the exciting active material, but decays with the time. The activity lasts for several hours when due to radium and several days when due to thorium. This property was first observed by M. and Mme. Curie[269] for radium, and independently by the writer[270] for thorium[271]. If any solid body is placed inside a closed vessel containing an emanating compound of thorium or radium, its surface becomes radio-active. For thorium compounds the amount of excited activity on a body is in general greater the nearer it is to the active material. In the case of radium, however, provided the body has been exposed for several hours, the amount of excited activity is to a large extent independent of the position of the body in the vessel containing the active material. Bodies are made active whether exposed directly to the action of the radio-active substance or screened from the action of the direct rays. This has been clearly shown in some experiments of P. Curie. A small open vessel _a_ (Fig. 62) containing a solution of radium is placed inside a larger closed vessel _V_. [Illustration: Fig. 62.] Plates _A_, _B_, _C_, _D_, _E_ are placed in various positions in the enclosure. After exposure for a day, the plates after removal are found to be radio-active even in positions completely shielded from the action of the direct rays. For example, the plate _D_ shielded from the direct radiation by the lead plate _P_ is as active as the plate _E_, exposed to the direct radiation. The amount of activity produced in a given time on a plate of given area in a definite position is independent of the material of the plate. Plates of mica, copper, cardboard, ebonite, all show equal amounts of activity. The amount of activity depends on the area of the plate and on the amount of free space in its neighbourhood. Excited radio-activity is also produced in water if exposed to the action of an emanating compound.