Recoil of Radium C from Radium B. 115 



into radium C, the difficulty would be removed. But it must 

 be admitted that the possibility of the existence of an un- 

 detected a particle in this transformation seems remote in 

 view of other evidence. 



There still remaius the question whether all the excess of 

 activity on the front of the disk over that on the back at the 

 higher pressures is due to radium C projected directly on to 

 it, or whether some of it finds its way there by diffusion 

 after having been stopped by the residual gas left even 

 at the highest obtainable vacuum. To test this possibility 

 the following experiment was made : — According to the 

 procedure already detailed, a brass disk was fixed 1*4 centi- 

 metre above a radiating surface, but shielded from direct 

 radiation by a copper screen placed between the surface 

 and the disk so that any activity obtained on the disk 

 must have reached it by diffusion. The exposure lasted half- 

 an-hour in vacuo, and on removal the disk was found to 

 have a very small activity both on the front and back, the 

 activity on the two sides being almost exactly equal. This 

 showed that the excess of activity obtained on the front o£ 

 the disk over that on the back in previous experiments is not 

 due to diffusion, but to radium C projected directly on to the 

 disk. 



The Electrical Char a e of Radium C. 



During the course of this work several attempts have been 

 made to ascertain whether radium C becomes charged with 

 electricity at the moment of its formation from radium B. 

 Assuming that radium B emit- only (3 particles, a simple 

 calculation shows that if radium C is charged, a field of 

 500 volts per centimetre between the plate and disk (fig. 1) 

 should suffice to cause the recoiling atom to return to the plate 

 from which it started after travelling T ^y millimetre. 



All attemps to stop radium C when it recoils by an electric 

 field have failed, indicating that radium C when formed from 

 radium B either remains electrically neutral or, as is sug- 

 gested by the absorption experiments just described, the 

 energy of a recoiling atom of radium is, for some unknown 

 reason, greater than has been supposed. 



We have again to thank Professor Rutherford for his 

 kindness in supplying us with the necessary radium emanation 

 and also for his interest in this work. 



12 



