370 Dr. L. Silberstein on 



(atomic number 58) a continuous spectrum in the place 



where the lines of the " K " series should appear. It was 



thought that possibly the spectrum of this series might 



consist of a large number of lines which blended into one 



another, giving the appearance of a continuous spectrum 



and due to the layers of the crystal not being accurately 



enough spaced for these short radiations. The penetrating 



power of these radiations being so great, every layer in the 



thick crystal is almost equally active, while for the " L " 



series, most of the reflexion comes from the planes in the 



first millimetre. A number of thin rock-salt crystals about 



one millimetre thick were then used, with the result that the 



bands were not nearly so uniform in intensity as in the 



photographs with the thick crystal. The lines were broad 



and diffuse, and many were wavy, so that measurements of 



any accuracy were impossible. In any case, there were no 



clearly marked characteristics of the " K" series, even when 



the maximum potential on the bulb was 100,000 volts. 



According to Planck's relation, E = Ay, the a line should 



appear when the electrons have the energy of approximately 



58,000 volts, while according to the observations in the 



accompanying paper, this characteristic if present should 



appear at 74,000 volts. These spectroscopic observations, 



therefore, agree with the absorption results, the curves of 



which showed no evidence of being able to be resolved into 



a few exponentials representing characteristic radiations. 



In conclusion, the author wishes to express his best thanks 

 to Sir Ernest Rutherford for many suggestions in the course 

 of the above observations, and also for the privilege of 

 working in his laboratory. 



XXXVII. On Mutual Electromagnetic Mass. By L. Silber- 

 stein, Ph.D., Lecturer in Natural Philosophy at the 

 University of Rome *. 



THE electromagnetic mass of a system of two charges, 

 ei and e 2 , is of the form 



m 1 + ?»2+??i 12 , 



where m 1? ra 2 are determined by the shape and size of, and 

 the distribution of electricity in, 1 and 2, respectively; 

 whereas m 12 depends on the mutual relations of the two 

 charged regions to one another. The first two terms of the 

 above sum being called the masses of 1 and 2, respectively, 



* Communicated by the Author. 



