2S4; Sir E. Rutherford and Messrs. Robinson ^* Rawlinson: 



that of the primary rays, where each group consists of rays- 

 of the same speed, but is very similar to the spectrum 

 observed when the primary ft rays have passed through an 

 absorbing screen. 



With a lead-absorbing screen, the most prominent groups 

 of excited ft rays were observed in that part of the spectrum 

 which corresponded to the strong groups of primary rays 

 from radium B, viz. those having velocities between 0*6 and 

 0*8 of the velocity of light. The photographs Avere prac- 

 tically identical in appearance, whether the tube was covered 

 with 01 or 1 mm. of lead, showing that a thickness of 0*1 mm. 

 w T as sufficient to cut out entirely the effect of the primary 

 ft rays. When the tube was covered with thin lead, evidence 

 was obtained of groups, of very feeble intensity, corre- 

 sponding approximately in position with the strong groups- 

 of primary rays from radium B marked E and F in the list 

 of lines given by Rutherford and Robinson. These are rays 

 having velocities approximately 0'4 of that of light. 



The values of the velocities of the excited ft rays were- 

 determined by direct comparison of the positions of the 

 groups with those of the stronger lines due to the primary 

 rays from radium B. For this purpose, an a-ray tube 

 containing about 10 millicuries of radium emanation was put 

 in the place of the first tube, and the primary /3-ray spectrum 

 obtained with the same magnetic fields. Small corrections 

 were necessary for the reduction in velocity of the primary 

 ft rays in passing through the walls of the a-ray tube, and. 

 also for the increase in dimensions of the radiating source 

 when surrounded by a metal cylinder. With thicknesses of 

 lead of about 1 mm. ; the latter correction is difficult to make 

 with certainty, for account has to be taken of the fact that 

 the excited ft rays are mainly projected in the direction 

 of the exciting 7 rays, and are thus most concentrated in the- 

 direction normal to the absorbing cylinder. 



The experiments with lead as the absorbing screen are of 

 special interest, for it has been shown by Rutherford and 

 Andrade in a previous paper that the spectrum of the soft 

 7 rays from radium B corresponds to that of the " L " 

 characteristic radiation of lead. It would thus appear very 

 probable that radium B and lead have the same nucleus 

 charge and the same external distribution of electrons- 

 although their atomic weights are different. If the primary 

 ft rays from radium B result from the conversion of 7 rays 

 into ft rays in their escape from the radioactive atom, it is 

 to be anticipated that the spectrum of the ft rays excited in 

 lead should be identical with that given by the primary 



