SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
435 
become radio-active. Some ignited yttrium sulphate, in the condition most sensitive 
to the cathode rays, and giving under their action a phosphorescent glow with a 
discontinuous spectrum, was pressed tightly into a shallow aluminium tray and 
exposed for an hour to the discharge in the vacuum tube. It was then removed 
and immediately covered with a sensitive film pressed down with a slight weight. 
After forty-eight hours in total darkness the film was developed. Only a very 
slight image of the yttria was visible. This result does not prove that the cathode 
stream had rendered the yttria radio-active, for there is always a residual glow 
in the yttria in these circumstances ; it is not unlikely that the light of this glow, 
acting for the first hour or so of the forty-eight, might have been strong enough 
to impress the film (7). 
7. Three shallow trays were filled—one with phosphorescent calcium sulphide, 
another with zinc sulphide, and a third with platinocyanide of barium. After 
subjecting them to the cathode stream in the above manner, they were removed 
and covered with a sensitive film, and kept in darkness for twenty-four hours. 
On development only an impression of the zinc sulphide was seen. As this sulphide 
also has a certain amount of residual glow after exposure to the cathode rays it 
is probable that the photographic action is only the result of this glow (6). 
Action of Radium on Diamond. 
In June, 1904,* I read a paper before the Royal Society on the action of Radium 
on Diamond. 
8. A few years later I repeated the original experiment, exposing two New South 
Wales diamonds to the action of the radium rays and emanations for a longer period. 
The diamonds selected were of an identical pale yellow, devoid of radio-activity. A 
quartz tube, containing 15 mgrms. of pure radium bromide was well exhausted and 
sealed before the oxy-hydrogen blowpipe. One of the diamonds (fig. 3 a) was put close 
to the tube of radium and kept in its place with a piece of gummed paper. The other 
diamond (fig. 36) w T as put away in a cabinet and kept far from any radium compound. 
The two diamonds were thus left for a little more than six months. At the end of 
this time they were examined. No appreciable difference could be detected in the 
colour of the two diamonds—the one (fig. 3a) that had been close to the tube of radium 
bromide not being darker than the one (fig. 36) which had been away from radium 
the whole time. 
Coloration of Diamond by a-Ray. 
9. The diamond, 3a, was now enclosed for seventy-eight days in a tube containing 
radium bromide ; at the end of the time it had acquired a bluish green colour. It was 
* ‘ Roy. Soe. Proc.,’ vol. Ixxiv., p. 47. 
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