108 Dr. W. Makower and Dr. S. Russ on the 



the recoil of radium B from radium A deposited on a plate. 

 In the case, however, of the recoil of radium C from 

 radium B, which is much less energetic, it was at once seen 

 that the fraction of the total number of particles of radium C 

 escaping from an active plate to the total number formed 

 was very small. This was clear without accurate measure- 

 ment for, whereas the activity of the plate could be measured 

 in a 7-ray electroscope, the activity on a disk exposed to it 

 for 20 minutes could be measured only by careful observations 

 with a sensitive a-ray electroscope. It was therefore 

 necessary to carry out a series of quantitative measurements 

 to determine the fraction of the number of recoils of 

 radium which resulted in its removal from the surface 

 of the plate. For this purpose, the activity of the radiating 

 plate was compared with that of a standard quantity of 

 radium by means of a y-ray electroscope in the usual 

 manner. The activity on a disk exposed in vacuo at a known 

 distance from the plate for 20 minutes, was measured on a 

 sensitive a-ray electroscope and compared with that of a 

 polonium standard * giving out a known number of u particles 

 per second. In this way the number of « particles emitted 

 from the active disk would be found and the number of 

 atoms of radium C on it deduced. 



In a long series of experiments in which the activity of 

 the radiating plate was equal to that of *2 milligram of 

 radium bromide, the quantity of radium C collected on a disk 

 suspended 2*5 cm. above it for 20 minutes in vacuo varied 

 between the equivalent of 5 x 10~ 7 and 1*4 x 10 -5 milligram 

 of radium bromide. Now, if half the radium C particles 

 produced had been projected upwards from the plate, the 

 disk, which was of diameter 1*7 cm., should have had on it 

 the number of radium C particles equivalent to about 4 x 10 ~ 3 

 milligram of radium bromide at the end of the exposure. 

 Thus the quantity of radium C reaching the disk varied from 

 8 Wo t° boo °^ *he total quantity which could have reached 

 it. The smallness of the amount of radium C escaping from 

 an active surface by recoil has recently been noted by Hahn 

 and Meitner f, who obtained little more than one millionth 

 of the total obtainable quantity of radium C on a plate 

 exposed to a surface covered with rndium B. This is a far 

 smaller quantity even than we have found ; but the experi- 

 ments of Hahn and Meitner were carried out at atmospheric 



* This standard was kindly lent to us by Dr. Geiger, who determined 

 the number of a particles emitted by it per second. 



f Hahn & Meitner, Phys. Zeitschr. x. 1909, pp. 697-703. 



