THE TRANSFORMATION OF ELEMENTS 77 



directly ; a ground-in joint was made (at o, fig. 2). Silica tubing 

 possesses several advantages over glass for such measurements. 

 Although the silica tube had less than half the diameter of the 

 glass tubes, yet the sticktion of the mercury was negligible, while 

 in the glass tubes it was so great that accurate readings could 

 be obtained only with difficulty. Again, silica is less readily 



TaPUMP: 



Fig. 2. 



coloured than ordinary glass, pure silicon dioxide being un- 

 affected. The first two capillary tubes were of potash glass, 

 which is so much affected by the /3 rays that after one or two 

 days the level of the mercury inside can no longer be observed. 

 The effect was partly overcome by confining the emanation to 

 the top of the tubes, except when actual readings were being 

 taken. 



