SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
441 
exposed to radium emanation (33) for forty-eight hours ; they were then removed 
from the chamber and laid upon a sensitive film for twenty-four hours. Upon 
development each object was found to be active and there was nothing to show that 
the three diamonds had acted differently to the quartz or glass (fig. 5). The objects 
were now all boiled in dilute nitric acid and thoroughly washed in clean water. They 
were placed on a film and allowed again to remain for twenty-four hours; on 
development it was seen that practically the whole of the “activity” had been 
removed—the two diamonds that originally were very slightly active had, perhaps, 
gained a little, but all the other objects had lost their temporary activity. 
32. The experiments were still further extended. Two fragments of lead glass 
and one diamond crystal that had been buried in radium for over two years and had 
become deeply coloured, the diamond green and the lead glass brown, were removed 
from the radium and cleaned as far as possible with a dry brush. They were kept 
upon a film for five days. On development the photograph showed three equally 
dense black patches. The glass and diamond were now boiled in dilute nitric acid, 
thoroughly washed, laid upon a film, and kept there for eighteen hours. Upon 
development the pieces of glass were seen to have almost lost their activity—the 
diamond gave a dense black impression. The objects were now placed in a mixture 
of fuming nitric acid and potassium chlorate and kept just short of boiling point for 
six hours, frequently removing the weakened solution and adding fresh acid and 
chlorate (3, 9, 12); they were then thoroughly cleaned, dried, and laid upon a film 
for eighteen hours. On development the glass blocks gave impressions slightly 
fainter than before, while the diamond still gave a dense black image almost as strong 
as before the treatment. 
33. An examination with a zinc-sulphide screen showed that the diamond caused 
brilliant scintillations from its edges and corners and a few erratic scintillations could 
be detected round each of the fragments of glass (11, 17, 26, 27, 30). The net result 
of these experiments is to show that although the well-known condensation of 
emanation (3l) upon crystals and objects gives rise to photographic markings of 
patterns as described in this research and in previous papers by Sir Ernest 
Rutherford* and other observers, this superficial activity can be easily and com¬ 
pletely removed by washing in dilute acids. The two kinds of acquired activity are 
illustrated in figs. 6 and 7. Fig. 6 is a radiograph of a crystal of quartz that had 
been rendered active by exposure to radium emanation, while fig. 7 is a radiograph of 
a crystal of diamond that had been covered in a bottle with radium bromide crystals 
for many months. While the former points to a superficial coating of active matter, 
the latter is more suggestive of a brush discharge from the corner of the diamond 
crystal. If the action of the emanation is allowed to continue until colour changes 
are produced both in glass and diamond, the activity is retained even after many 
* “ Distribution of the Intensity of the Radiation from Radio-active Sources,” by Sir Ernest 
Rutherford, F.R.S., ‘Phil. Mag.,’ August, 1906. 
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