﻿Positive Thermions from Salts of Alkaline Earths. 121 

 plate, then using a spark between aluminium terminals, 



For distance 37'6, j- = 1-82, 



For distance 110*2, ~ =1; 



and for iron terminals, 



F 

 For distance 37'G, =p = l' : 7'8, 



F 

 For distance 1102, j- = ii 



The difference in the two cases is due to the fact that the 



spectrum of the light emitted by the aluminium is different 



from that of the iron. But in both cases it is apparent that if 



/ 37*6 Y . 1 

 the intensity is decreased by the factor ( 9 i , t. e. - about ; 



there is a very large decrease (45 per cent.) in the ionizing 

 power. With the Rontgen rays, even taking the isolated 

 readings which give the extreme values and taking the rays 

 to diverge from the anode, a decrease of -^ only produces a 

 change of 13 per cent. 



If then the forces in a Rontgen ray pulse are in any way 

 comparable to those in a beam of ultra-violet light, the results 

 of this experiment discredit to some extent the pulse theory; 

 while establishing, as they appear to do, the uniformity of the 

 ionization per cm. in a beam of rays, they remove a possible 

 objection to the corpuscular theory. 



This research was conducted in the Electrical Laboratory 

 at Oxford, and I should like to express my thanks to 

 Professor Townsend for much assistance and encouragement. 



X. The Positive Thermions from the Salts of the Alkaline 

 Earths. By Clinton J. Davisson, B.S., Fellow in Physics, 

 Princeton University, Princeton, N.J., U.S.A.* 



Introduction. 



THAT positive electrical charge is emitted by many 

 chemical compounds when sufficiently heated was 

 discovered by J. C. Beattie t in 1809. Since then numerous 

 experiments have been made to determine the laws of this 



* Communicated bv Prof. 0. W. Richardson. 

 t Phil. Mau-. July 1899. 



