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XIII. On Acquired Radio-Activity. 
By Sir William Crookes, O.M., LL.D., D.Sc., Pres.R.S. 
[Plates 9-10.] 
Received October 26, 1914. 
Experiments with Cathode Rays. 
1. About forty years ago I sealed in a vacuum tube a yellow diamond cut as a 
brilliant. This diamond was chosen because it phosphoresced in the dark after 
exposure to bright sunshine,—-it also phosphoresced slightly under the influence of 
ultra-violet light. In the vacuum tube, as the anti-cathode, it emitted a brilliant 
yellowish white light giving almost as much light as a candle. It was often 
exhibited to illustrate the glow of a diamond under the influence of cathode rays 
in vacuo; scarcely a week passed without the vacuum tube containing the diamond 
being exhibited to friends. It was by far the most attractive tube in my 
collection. After forty years of hard work the diamond has become much dis¬ 
coloured. It was of interest to see if the repeated bombardment, as the anti-cathode, 
in the vacuum tube had conferred radio-activity on the diamond. Accordingly 
I opened the tube, quickly removed the diamond, and in the dark put it on a sensitive 
film, a thin sheet of black paper intervening. Over the diamond was placed a pad of 
cotton-wool and a weight, to prevent the stone from shifting its position. After nine 
days’ contact the film was developed. An exceedingly slight action could, with 
difficulty, be detected, showing that the off and on action for forty years had 
conferred practically no radio-activity on the diamond. 
2. A sensitive electroscope was now made, with sulphur insulation. The fall of the 
leaf was observed with a small telescope having a photographed scale. The normal 
fall due to natural leakage was 5" in 165 seconds. A piece of a thoria gas mantle 
caused a fall of 5° in two seconds, and radio-active diamonds caused a fall in 
from two or three seconds to a fraction of a second. Tested in this instrument 
I found this special diamond to be entirely devoid of action. 
Action of Cathode Rays on Diamond. 
3. On one occasion when M. Moissan was in my laboratory I darkened some 
diamonds by means of cathode rays. One of these he took away, and subsequently 
reported the result of his experiments to the French Academy.* He heated 
* ‘Comptes Rendus,’ vol. cxxiv., No, 13, p. 653, March, 1897. 
VOL. CCXIV.-A. 521. 3 K Published separately, December 1G, 1914. 
