444 
SIR W. CROOKES ON ACQUIRED RADIO-ACTIVITY. 
tint, the other, smaller, was only just luminous. A sheet of aluminium, 3'2 mm. 
thick, was now interposed in the path of the X-rays, when the phosphorescence of 
the large bright stone was considerably reduced, while that of the small faint 
stone was not diminished at all. 
Action of /3-Rays on Diamond. 
These two diamonds were subjected to the action of solid radium bromide, and 
the intensity of the phosphorescence thereby induced was seen to be of the same 
character as that caused by the X-rays. An extension of the experiment with 
radium, described in the next paragraph, showed that the effect of phosphorescence 
was mostly due to the /3-rays (10, 19, 26). 
43. An apparatus (fig. 17) was fitted so that the active rays from a bottle of 
radium bromide (a) should pass through a tube 
drilled through a lead block (b), and allowed to 
pass upwards on a diamond (c) supported on a 
plate of aluminium, 0'02 mm. thick. This was 
arranged between the poles of a powerful electro¬ 
magnet, so that the active rays from the radium 
should pass through the hole and act on the 
diamond when the magnetism was off, and be 
deflected from it when the magnetism was on. 
A screen of barium platinocyanide put over the Fig. 17. 
hole in the lead block showed a circular spot of 
phosphorescence. This luminosity vanished when magnetism was turned on, and 
reappeared when it was turned off, showing that the luminosity on the screen 
was chiefly due to the /3-rays (10, 19, 26). The two large and small diamonds 
used in the last experiment were put side by side on the aluminium, and the 
support so arranged that it could be moved sideways to put either stone over 
or away from the hole in the lead block. It was easy to see by small movements 
of the support that, as in the case of the X-rays, the large stone was much 
more phosphorescent than the smaller stone. There was considerable residual 
luminosity, each of the stones continuing to glow with its relative intensity when 
effectually removed from the radium. Having found the position of maximum 
brightness for the large stone when over the radium, the current was turned on, 
so as to deflect the /3-rays. At once the glow declined, and the residual phos¬ 
phorescence faded. On taking the current off, so as to allow the /3-rays again 
to come into action, the brilliancy increased again. There was no doubt as to the 
action, although the undeflected y-rays and the residual luminosity tended to 
obscure the observations. 
