356 Sir E. Rutherford, Prof. Barnes, and Mr. Richardson : 



radiations, and also some comparative results obtained with 

 the Coolidge tube. 



Table III. 



Wave-length of 



strong lines 

 from Radium B. 



1-37x10 



159 „ 



1-69 „ 



1-96 „ 



2-29 „ 



9-82 „ 



to 



11-75 „ 



9 cm. 



Absorption coeffi- 

 cient of radiations 

 from Radium B. 



1-5 (cm.) 



46 

 •51 



Pb. 



in Al. 



Wave-length of 

 radiation from 

 Coolidge tube. 



Absorption coeffi- 

 cient of radiations 

 from Coolidge tube. 



1-72x10 cm. }. 

 1-91 X 10 ~ 9 cm. 



"L " radiation. 

 =40 in Al. 



1082 



to _ 9 



14-77x10 cm. 



23 (cm. 



:36 



:'45 . 



in Pb. 



in Al. 



" L " radiation. 

 At = Sl in Al. 



The line in radium B, \ = 1*96 x 10~ 9 , appears to be mainly 

 responsible for the radiation which has an absorption yu- = *51 

 in aluminium and /i,=46 in lead. This is clear from a 

 comparison of the results with the Coolidge tube. Taking 

 Moseley's observation that the frequency of the correspond- 

 ing lines in the K radiations of different elements is pro- 

 portional to (N — l) 2 where N is the atomic number, it 

 follows that the line in the radium B spectrum for which 

 X = l-37xl0- 9 should have a value X = l-69xl0" 9 for 

 tungsten, since the latter has an atomic number 74 and 

 radium B, 82. This calculated value is in good agreement 

 with the maximum wave-length \— 1*73 X 10~ 9 found for 

 tungsten. It thus appears probable that the radiation from 

 tungsten is analogous to the radiation from radium B. Since 

 the speed of the beta rays issuing from radium B corresponds 

 to a fall of potential of at least 400,000 volts, and from 

 radium C of 2,000,000 volts, it seems clear that we cannot 

 expect to obtain a more penetrating radiation from tungsten 

 unless possibly a voltage of the order of 1,000,000 volts 

 is applied. Even with electrons corresponding in energy to 

 over 2,000,000 volts, the wave-length of the strong line 

 due to the penetrating gamma rays from radium C, viz. 

 A.=-99xl0" 9 , is only 6/10 of the shortest wave from the 

 Coolidge tube. The comparison of the results with the 

 Coolidge tube with the gamma rays thus leads to the conclu- 

 sion that there is a definite limit to the maximum frequency 

 to be obtained from an element bombarded by swift electrons. 



