DR. CHARLES O. BARKLA ON POLARISED RONTGEN RADIATION. 477 



in intensity of the secondary radiation by rotating the bulb was very much smaller 

 than in the cases previously considered. In fact, with the metals experimented upon 

 it could not be detected. 



Using copper as the radiator, the almost perfect agreement between the relative 

 deflexions of the secondary electroscopes in the two positions of the bulb is shown in 

 Table IV. 



Tin and iron also gave results similar to those obtained with copper. They are 

 shown in Tables V. and VI. 



TABLE VI. 



These results considered in connection with the fact that the secondary radiation 

 from heavy metals differs considerably in character from the primary producing it are 

 significant. In light substances, on the electronic theory, the distances between the 

 electrons are greater than in the heavier substances. It seems possible that this is 

 the immediate cause of the difference, for when the distance apart becomes so small 

 as to be comparable with a pulse thickness the theory is not such a simple one as 

 that given, as there are mutual actions between the electrons. Other possibilities 

 might be discussed, as that of the vibration frequency in the heavier atoms being 

 such as to cause a radiation having ionizing power not possessed by that proceeding 

 from the electrons of lighter atoms, or of a temporary radioactivity being set up in 

 the heavier atoms by the transmission of X-rays. Experiments are being made on 

 the subject, and will be treated in a later paper on Secondary Radiation from Metala 



With copper as secondary radiator, no evidence of a change in intensity of secondary 

 radiation, when the position of the X-ray bulb was changed in the manner described, 

 was given. It was thought possible that by using more penetrating radiation for 

 the primary beam that is, on the generally accepted theory, a radiation consisting 



