440 
SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY 
on the blende screen (17, 26, 30, 33), giving forty-eight hours’ exposure (fig. 3a). 
The three bunches of luminosity come from the corners of the crystal that are in 
contact with the screen. The photographic impression is certainly discontinuous, 
appearing granular, corresponding to the granular character of the screen. It 
appears that the different grains shine and continue to shine by their own residual 
phosphorescent light under the impact of the a-particles. In a further experiment 
the diamond was illuminated by the arc lamp, and a photograph was made of the 
diamond crystal in situ as well as of the screen. This gave a good image of the 
stone, and also of the granular surface of the screen (fig. 3 h). 
28. It is certain that, in addition to a-rays, the diamond (17) gives off a consider¬ 
able amount of other rays (10, 19, 26, 42, 43). This is shown by the early experi¬ 
ments, where photographic images with the diamond were obtained through one, 
two, three, and four sheets of aluminium foil—each O'Ol mm. thick. 
29. The crystal of active diamond (17), with which most of these experiments 
have been made, was examined crystallographically by Dr. Tutton, who reported to 
me that the crystal is an apparent octahedron—but composed of two supplementary 
tetrahedra showing on three of the edges the usual grooves where the interpenetration 
of the tetrahedra is not complete. 
Each of the eight sides of the diamond was photographed by its own radio-activity 
(%• 4 ). 
30. Experiments show that exposure of a zinc-sulphide screen to the impact of 
electrons from the negative pole in a vacuum tube does not cause scintillations. It 
appears that only a-rays (positive atoms) produce this efiect (l 1, 17, 26, 27, 33); it 
was therefore interesting to see if scintillations could be produced by bombarding a 
sulphide screen with a stream of positive atoms produced in a “ Thompson tube. 
An apparatus was fitted up having a zinc-sulphide screen to receive the positive 
discharge—but no effect of scintillation could be observed. 
Action of Radium Emanation on Diamond and on other Bodies. 
31. After repeated .observations on the action of radium on various substances, it 
was seen that diamond behaved differently to glass, quartz, and other materials, 
especially if the action of the radium had been continued for some length of time. 
The following experiments were made to see if definite light could be thrown upon 
this point. Some plates and crystals of pure quartz, lead glass, Faraday’s boro- 
silicate lead glass, and three diamonds were selected—well cleaned, and exposed upon 
a sensitive film for twenty-four hours. On development it was seen that all were 
inactive, except in the case of two of the diamonds, but from these the action was so 
slight as to be only just visible upon the film. These objects were now laid upon a 
table of clean platinum gauze and enclosed in an air-tight glass vessel, together with 
an open bottle containing pure radium bromide. They were allowed to remain 
