SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
436 
then heated for ten days in a mixture of* fuming nitric acid and potassium chlorate so 
as to dissolve off any outer skin of graphite which might have contributed to the 
colour (3, 12, 32). This treatment brightened its appearance but did not alter the 
colour. The diamond was next put on a sensitive photographic film and kept there 
for twenty-four hours. On developing, a strong impression was seen. 
ft- and y-Rays produce Phosphorescence. 
10. This experiment shows that the alteration of the colour is not due to the 
phosphorescent state of excitement to which the diamond had been constantly 
subjected during eight weeks. The colouring action is cut off by a thin screen of 
quartz, whereas the phosphorescing action is kept up by rays which pass through 
quartz. It is therefore evident that the coloration is due to the a-rays, or atoms of 
helium, shot from the radium compound (26, 29, 33). The phosphorescence is 
produced by the ft- or y-rays (19, 26, 43), which readily pass through glass and 
quartz. 
Persistence of Acquired Radio-activity. 
11. The acquired radio-activity of diamonds persists for a longer time than I have 
been enabled to measure, and resists the most violent treatment I have applied to 
them. 
A large brilliant cut diamond of pure water assumed a fine green colour after having 
been kept for sixteen months (from May, 1904, to Sept., 1905) in a bottle and covered 
with powdered radium bromide. At the end of that time it was highly radio-active. 
This diamond has been carried about in my pocket, off and on, since 1905, and has 
been tested on a sensitive photographic film at intervals of a year or more. No 
appreciable difference in its radio-activity can be detected from that which it 
possessed when first removed from the radium bromide in Sept., 1905. Examined 
at the present time, nine years after its removal from the bottle of radium bromide, 
it is luminous in the dark, it rapidly discharges a sensitive electroscope when held 
near it, and produces scintillations on a zinc sulphide screen as if it were a radium 
compound. 
12. A diamond of good water was selected from a stock of inactive stones, and put 
m a tube of radium bromide for seventy-eight days. At the end of that time it had 
assumed a greenish colour, and was highly radio-active. It was then heated to 50° C. 
in a mixture of fuming nitric acid and powdered potassium chlorate for ten days, the 
acid mixture being daily renewed (3, 9, 32). The only effect of this acid treatment 
was to remove a slight dull darkening on the surface, and to render the green tint 
more brilliant. 
