164 PHYSICS OF THE ELECTRON 



of rays are transverse pulses set up in the ether by the sudden arrest 

 X the motion of the cathode particles on striking an obstacle. The 

 more sudden the stoppage, the shorter is the pulse, and the rays, in 

 consequence, have greater power of penetrating matter. In some 

 recent experiments Barkla found that the secondary rays set up 

 by the X rays, on striking an obstacle, vary in intensity with the 

 orientation of the X-ray tube, showing that the X rays exhibit the 

 property of one-sidedness or polarization. This is the only evi- 

 dence so far obtained in direct support of the wave-nature of the 

 X rays. 



If X rays are not set up when the cathode particles are 

 stopped, conversely, it is to be expected that X rays should be set 

 up when they are suddenly set in motion. Now this effect is not 

 observable in an X-ray tube, since the cathode particles acquire 

 most of their velocity, not at the cathode itself, but in passing through 

 the electric field between the cathode and anti-cathode. It is, how- 

 ever, to be expected theoretically that a type of X rays should 

 be set up at the sudden expulsion of the /? particles from the radio- 

 atoms. The rays, too, should be of a very penetrating kind, since 

 not only is the charged particle projected with a speed approaching 

 that of light, but the change of motion must occur in a distance 

 comparable with the diameter of an atom. 



On this view, the 7- rays are a very penetrating type of X rays, 

 having their origin at the moment of the expulsion of the /? particle 

 from the atom. If the /? particle is the parent of the f rays, the 

 intensity of the /? and f rays should, under all conditions, be propor- 

 tional to one another. I have found this to be the case, for the f rays 

 always accompany the/? rays and, in whatever way the /3-ray activity 

 varies, the activity measured by the f rays always varies in the same 

 proportion. Active matter which does not emit /? rays does not give 

 rise to 7- rays. For example, the radio-tellurium of Marckwald, which 

 does not emit /? rays, does not give off f rays. 



Certain differences are observed, however, in the ionizing action of 

 f and X rays. For example, gases and vapors like chlorine, sulphur- 

 etted hydrogen, methyl-iodine, and chloroform, when exposed to 

 ordinary X rays, show a much greater ionization, compared with air, 

 than is to be expected, according to the density law. On the other 

 hand, the relative ionization of these substances by f rays follows the 

 density law very closely. It seemed likely that this apparent differ- 

 ence was due mainly to the greater penetrating power of the f rays. 

 This was confirmed by some recent experiments of Eve, who found 

 that the relative conductivity of gases exposed to very penetrating 

 X rays from a hard tube approximated in most cases closely to that 

 observed for the f rays. The vapor of methyl-iodine was an excep- 

 tion, but the difference in this case would probably disappear if 



