CH. IX] EXCITED RADIO-ACTIVITY 251 
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 ac- 
tivity 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 1s 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. 47) containing a solution of radium is placed inside a larger 
closed vessel V. 

Fig. 47. 
Plates A, B, C, D, # 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 #, 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 
