SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
445 
44. The results described in this paper may be summarised as follows :—-Various 
objects, diamond, ruby, garnet, quartz, gold, platinum, &c., also the phosphorescent 
substances yttria, calcium sulphide, zinc blende, and barium platinocyanide, are 
bombarded in a high vacuum by cathode rays, and in no case can any permanent 
activity be recognised either by photographic or electrical means (l, 2, 3, 4, 5, 6, 7). 
Exposure to radium emanation confers temporary radio-activity on all bodies 
that have been tried ; apparently due to the condensation of the emanation on the 
surface. This transient activity can be completely removed by washing in dilute 
acids (3l). 
Many substances become coloured by direct exposure to radium, the colour 
depending on the substance. Diamonds take a full sage-green tint, the depth 
depending on the time of exposure to the radium (33). 
In addition to change of colour diamond also becomes persistently radio-active 
continuously giving off a-, 6-, and y-rays. The acquired colour and activity with¬ 
stand the action of powerful chemical agents, and continue for years with apparently 
undiminished activity (12, 31, 32). 
Removing the surface by mechanical means removes both colour and radio¬ 
activity (13, 16, 35, 37, 41). 
The appearance of an auto-radiograph made by placing an active diamond crystal 
on a sensitive photographic plate, and the visual examination of its “scintillation” 
luminosity, suggest that there is a special discharge of energy from the corners 
and points of the crystal (18, 21, 23, 27, 33). 
