Recoil of Radium C from Radium B. Ill 



velocity to be 1*77 X 10 9 centimetres per second *, then, from 

 the equation of momentum, the velocity with which the 

 particle of radium B must be travelling after recoil will be 

 3*25 x 10 7 centimetres per second. It is evident that an atom 

 of radium A when formed from the emanation will recoil with 

 approximately the same velocity. On the other hand, since 

 the mass of an atom of radium B is 214 and that of a 

 /3 particle Tyuo"* then, if we assume the velocity of the 

 /3 particle from radium B to be 10 10 centimetres per second f , 

 the velocity of expulsion of the radium C atom formed by the 

 process will be 2*75 X 10 4 centimetres per second. It therefore 

 follows that an atom of radium B will recoil with 1*39 x 10 6 

 times the energy of an atom of radium (J. The penetration 

 of matter by radium C should therefore be less than one- 

 millionth or! that of radium B, if the power of an atom of: 

 penetrating matter is proportional to its energy. 



Now the latter radiation is exceedingly easily absorbed, 

 being cut down to one-tenth of its initial value by passing 

 through 6*5 centimetres of air at a pressure of one millimetre 

 of mercury. It might therefore be expected that even a 

 minute quantity of air would be sufficient to entirely stop 

 radium U from escaping any distance from a deposit of 

 radium B. This is, however, not the case ; for the radium 

 expelled by recoil from radium B can penetrate appreciable 

 quantities of air before being stopped, and it was even found 

 possible to measure its absorption by air, though the experi- 

 ments were difficult and the accuracy of the results not great. 



The apparatus used was that shown in fig. 1. In view of 

 the very large absorption to be expected, and on account of 

 the necessity of working with small quantities of radium B in 

 order to get pure radium C projected on to the disk B, it was at 

 first thought desirable to bring the disk close to the radiating 

 plate. The first experiments were therefore made with the 

 disk suspended 2 millimetres above the active plate, and the 

 radiation received by the disk after an exposure of 20 minutes 

 to the plate measured, the pressure of the air in the vessel V 

 being varied. It was, however, soon found that working in 

 this wav very irregular results were obtained, and it seemed 

 that the disturbances were produced by secondary causes due 

 to the diffusion of the radium C particles when stopped by 

 the air ; for if a certain proportion of the atoms of radium 

 projected from the active plate are stopped by impact with 



* Rutherford, Phil. Mag. Oct. 1906. 



t There appear to be no available data regarding the velocity of the 

 /3 particles emitted by radium B ; but the value assumed is such as to 

 give an upper limit to the velocity of expulsion of radium C. 



