Spectrum of ft Rays excited in Metals by Soft X Rays. 277' 

 = 10'36. With high voltages the intensity of the radia- 

 tion emitted fluctuated considerably, and it was difficult to 

 determine the complete absorption curve with accuracy. It 

 was found that the radiation became more and more pene- 

 trating as the voltage was increased. Assuming that it was 



exponentially absorbed, it was found that the value of - for 



the hard radiation was about 3'25 for 42,000 volts, and 2*54 

 for 51,000 volts. No doubt if the voltage could have been 

 pushed higher, the radiation would have become still more 

 penetrating. 



A number of experiments were made to examine whether 

 this penetrating radiation gave aline spectrum when reflected 

 from rock-salt. No conclusive evidence of definite fre- 

 quencies in the radiation was obtained. 



In conclusion I have great pleasure in expressing my 

 thanks to Sir Ernest Rutherford for suggesting these ex- 

 periments and for his interest throughout the course of the 

 work. 



University of Manchester, 

 June 30, 1914, 



XXXIII. The Magnetic Spectrum of the ft Mays excited in 

 Metals by Soft X Rays. By H. Robinson, M.Sc, and 

 W. F. Rawlinson, B.Sc, University of Manchester * '. 



IT has long been known that when soft X rays fall on 

 matter they give rise to ft or cathodic particles, the 

 velocities of which are dependent on the hardness of the rays. 

 A large number of interesting experiments on this question 

 were made several years ago by P. D. Innes f, who examined 

 by a photographic method the range of velocities of the 

 ft rays excited in various metals. He found relatively small 

 ranges of velocity, the greatest ranee — 6*3— 8'3xl0 9 cm.. 

 per sec. — being obtained with hard X rays impinging on 

 lead. The minimum velocity of the escaping ft rays was 

 found to be very nearly the same for all the metals used,, 

 while the maximum velocity increased both with the hardness 

 of the tube and with the atomic weight of the metal. 



The great increase in our knowledge of the X rays, derived 

 from the study of their reflexion by crystals, has made it 

 desirable that the experiments of Innes should be repeated 



* Communicated by Sir Ernest Rutherford, F.li.S. 

 f Innes, Proc. Roy. Soc. A. Ixxix. p. 442 (1907). 



