442 
SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
hours boiling in powerful acids, and the diamond retains its activity with much 
greater tenacity than the other materials—nothing short of actually cutting away 
the surface will remove it. The diamond crystal—that had been rendered active by 
immersion in dry radium bromide for some months in 1906 (17) and has been treated 
with acids, heated, and used in hundreds of experiments since—-is apparently as 
active now as when it was removed from the radium, and still pours out streams of 
a-rays # (Plate 10). Fig. 8 shows two radiographs of this crystal, having an interval 
of five years between them. Fig. 8 a was taken in 1909 and fig. 8 b a few days ago. 
34. As in the case of diamond, a quartz glass dish after the action of radium 
retains its colour and activity, and continues to give off a.-, /3~, and y-rays (fig. 9). 
One-half had a film of aluminium, 0'035 mm. thick, interposed between the dish and 
the sensitive plate. Vessels of pure fused silica that have been much used for the 
crystallisation of radium salts become coloured a purplish tint, similar to that 
assumed by soda glass in similar circumstances. On heating such a coloured vessel 
with a spirit lamp to a temperature much below a red heat it suddenly phosphoresces 
a bluish colour and at a red heat becomes colourless. 
35. A small equilateral prism was cut from a block of heavy glass (sp. gr. 3’87), 
polished on its three refracting faces, but left rough ground on the triangular top and 
bottom. It was put into a bottle and covered with dry crystals of radium bromide 
for fifteen hours. At the end of this time it was removed, well washed and placed 
on a sensitive film, base downwards, for twenty-two hours. On development a strong 
action was apparent with overflow radiation along the sides and the continuation of 
the faces in triangular lines (fig. 10) as observed by Sir E. Rutherford. 
36. One of the rough ground bases of the prism was now polished over the greater 
part of its surface, leaving a strip of glass along one edge in its original unpolished 
condition. This end was then laid on a sensitive film for thirty-six hours. On the 
polished portion of the surface there was absolutely no perceptible action on develop¬ 
ing, while the part left unpolished was highly active—as in the first experiment with 
the prism (fig. ll). 
37. The prism w~as now laid on a sensitive film on one of its f)olished faces for 
twenty-four hours. On development slight action was observed where the polished 
face touched the sensitive surface and a much stronger overflow action with 
continuation of the edges of the prism face in straight lines as mentioned above 
(fig. 12). 
38. The same prism was taken, and a diagonal scratch made with a writing 
diamond across the same face that touched the film in the last experiment. It was 
laid with this face downwards on a sensitive film for twenty-four hours. On 
* These experiments are closely connected with those of Swinton, Colley, Ramsay, and others, who 
have shown that helium is driven into the glass walls of a vacuum tube even by the slower moving 
cathode rays. The high speed a-particles evidently are driven into the diamond, below the molecular 
surface. 
