CHAPTER IX. 
EXCITED RADIO-ACTIVITY. 
165. Excited radio-activity. One of the most interesting 
and remarkable properties of thorium and radium 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®. 
SG 5 dity WAS. jon (Ales, llfete}e). 2 Phil. Mag. Jan. and Feb. 1900. 
® As regards date of publication, the priority of the discovery of ‘excited 
activity” belongs to M. and Mme Curie. A short paper on this subject, entitled 
‘Sur la radioactivité provoquée par les rayons de Becquerel,” was communicated 
by them to the Comptes Rendus, Nov. 6, 1899. A short note was added to the 
paper by Becquerel in which the phenomena of excited activity were ascribed to a 
type of phosphorescence. On my part, I had simultaneously discovered the 
emission of an emanation from thorium compounds and the excited activity 
produced by it, in July, 1899. I, however, delayed publication in order to work 
out in some detail the properties of the emanation and of the excited activity and 
the connection between them. The results were published in two papers in the 
Philosophical Magazine (Jan. and Feb. 1900) entitled ‘‘ A radio-active substance 
emitted from thorium compounds” and ‘‘ Radio-activity produced in substances by 
the action of thorium compounds,”’ 
