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[1] for radium, and independently by the writer[2] for thorium[3].
- ↑ M. and Mme. Curie, C. R. 129, p. 714, 1899.
- ↑ Rutherford, 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."
