Page:Radio-activity.djvu/317

 CHAPTER VIII.

EXCITED RADIO-ACTIVITY.

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 for radium, and independently by the writer for thorium.