Investigation of' the Nature of the 7 Rays. 671 
therefore have been part of the X-ray stream, and must start 
their independent existence by moving on in the line of the 
X-ray motion. Their velocity is much smaller than that of 
the secondary cathode rays due to 7 rays, and they are much 
more readily scattered. It may still remain an open question 
whether or no the X-ray stream contains ?ether pulses. 
Perhaps their existence must be supposed in order to explain 
the velocity experiment of Marx, and the diffraction experi- 
ment of Haga and Windt. Possibly they are also required 
in order to explain Barkla's polarization experiments ; but 
we do not think that the experiment described by Barkla in 
4 Nature ' (Oct. 31, 1907) is in any way decisive. 
It seems proper to consider a possibility that the negative 
particle, when it moves on in the original line of motion of 
the pair from which it came, retains also its original velocity. 
It is a striking fact that the cathode particle due to the 
7 ravs has the same speed, very nearly, as the ft particle 
issuing from the original radioactive material. And it looks 
quite unlike a coincidence that similar comparisons can be 
made in the case of the X rays. The secondary cathode 
radiations due to these rays have velocities which, at the 
least, are of the same order as the velocities of the cathode 
particles in the X-ray bulb. If we examine the table given 
by Innes (Proc. Roy. Soc. Aug. 2, 1907, p. 461), and if we 
may be allowed to adopt an interpretation differing somewhat 
from the author's, but more natural, it seems to us, in view 
of the conclusions of this paper, we find that the velocities 
of the electrons emitted by all the metals are practically the 
same, zinc being an exception because it is unable to break 
up the hardest rays. We find that the velocities range from 
about 6 x 10 9 to 7*5 x 10 9 for soft rays, and 6 x 10 9 to 8 X 10 9 
for hard rays. Remembering that bundles of X rays are 
very heterogeneous, the natural conclusion seems to be that 
the softest rays give the slowest speeds, and that the velocity 
of the secondary rays increases with the hardness of the 
X rays from which they are derived. Now the hardness of 
the rays grows with the speed of the cathode particles in the 
bulb. Is it then possible that the cathode particle is first 
set in motion by the electromotive force in the bulb, strikes 
the anticathode and picks up a positive there, becomes neutral 
and is now called an X ray, is subsequently stripped of the 
positive and becomes a secondary cathode particle, the identity 
of the negative remaining the same throughout and its speed 
invariable or nearly so ? The difficulty comes in when we 
try to consider the part played by the mass of the positive. 
And, again, may not the ft and 7 forms be interchangeable 
at times ? A 7 particle which had been stripped of ifr 
