162 Penetrating Power of X Radiation from Coolidge Tube. 



200,000 volts, and it is to be noted in the table that three 

 strong groups, B, C, and D, of ft rays from radium B corre- 

 spond to voltages between 261,000 and 152,000 volts, an 

 average of about 200,000 volts. The value of //,= 1*5 may 

 correspond to group A or a still swifter group, of voltage 

 about 500,000 volts, observed in the spectrum of ft rays 

 excited in lead by the gamma rays from radium B and 

 radium G together *- 



The results as a whole suggest that the groups of ft rays 

 are due to the transformation of the gamma rays in single 

 and not multiple quanta, according to the relation E = Az^ 

 The multiple relations observed between the energy of some 

 of the groups of ft raysf must on this view indicate 

 approximate multiple relations between the frequencies of 

 the gamma rays. 



With the assistance of Mr. J. West, B.Sc, I have made 

 some experiments to see whether it is possible to detect by 

 the crystal method the presence of waves shorter than those 

 observed in the experiments of Rutherford and Andrade 

 (loc. cit.). A narrow pencil of gamma rays and strong- 

 sources were employed, but no certain evidence of the 

 existence of such waves was obtained. This may be due 

 either to the overlapping of the numerous lines that should 

 be present, or to the failure of the crystal to resolve waves 

 whose length is very small compared with the grating 

 space. 



If the single quantum relation should prove to hold 

 generally for the conversion of 7 rays into ft rays, the mag- 

 netic spectrum of ft rays shorld afford a reliable method 

 of extending the investigation of X-ray spectra into the 

 region of very short waves where the crystal method 

 either breaks down or is practically ineffective, and thus 

 places in our hands a new and powerful method of analysing 

 waves of the highest obtainable frequency. The complexity 

 of the /3-ray spectrum for radium B and radium C indicates 

 that the spectrum of the gamma rays, and presumably the 

 very high-frequency spectra of heavy elements in general, 

 are as complicated as the ordinary light spectra of such 

 elements. 



University of Manchester. 

 May 12, 1917. 



* Rutherford, Robinson, and Rawlinson, Phil. Mag. xxviii. p. 285 

 (1914). 



t Rutherford, Phil. Mag. xxviii. p. 305 (1914) 



