932 Dr. J. CJhadwick and Mr. E. S. Bieler on the 



disk by immersion* in a strong solution of radiothorium. 

 Thorium gives complex a-rays, viz. 65 per cent, of range 

 S'6 cm., and 35 per cent, of range 5*0 cm. The number 

 of particles of range 8' 6 cm. emitted by ThC has been 



Fig. 4. 



compared recently in this laboratory by Professor Schlundt 

 and Mr. Shenstone with the number of a particles emitted 

 by Ra(B-}-C) of equal 7-ray activity. Under the con- 

 ditions of measurement hi our experiments, the number 

 of S'6 cm. particles per milligram activity of ThC is taken 

 as 0*75 of the number of 7 cm. a particles per milligram 

 activity of Ra(B + 0) . 



The H particles due to the a particles of range 5*0 cm. 

 were cut out by absorbing screens placed in front of the 

 ZnS screen. 



Sources of u particles of smaller range than 7 cm. were 

 obtained by placing gold or silver foils of known stopping - 

 power over a source of radium (B + C), as at F in fig. 2. 



The results are shown in curves A, C, and D of fig. 4. 

 It will be noted that the observed numbers of H particles 

 are greatly in excess of those which would be given by 

 point charges and the inverse square law of force. On this 



