ELECTRIC RADIATIONS—BRAGG. 211 
that is to say, differ only as regards their production of secondary 
ionization. Now, the a and @ rays are certainly material particles, 
possessing electric fields. There is, therefore, a reasonable argument 
that the y and X rays are also material, and possess electric fields. 
This is the case if they are pairs, and the smaller the moments are 
the more circumscribed are the fields and the less the ionization and 
the loss of energy. 
If the X rays contain ether pulses only, it is difficult to see why 
their effects should run so exactly in parallel with those of the a and 
B rays. 
It has been announced by Marx, as the result of a most ingenious 
experiment (Phys. Zeit., 1905, p. 268), that Rontgen rays move with 
the velocity of light. It is extremely improbable that material par- 
ticles can possess such a velocity, and 
the experiment of Marx might seem at 
first sight to be strongly against any 
material nature of the X rays. But it 
is not clear that Marx really measured 
the velocity of a radiation causing the 
emission of high-speed electrons, which 
is the characteristic feature of X rays. 
All that he showed was that the bun- 
dle of X rays contained radiation 
moving with the speed of light and 
capable of exciting S rays. To see this 
it is necessary to consider briefly the 
details of the experiment. 
An electric pulse is made to travel 
along a wire, W, as shown in the ac- 
companying sketch. When it reaches 
the cathode, C, cathode rays are driven Earth 
against the anode, A, and X rays are Fig. 1. 
given out, some of which travel toward the saucer-shaped electrode, 
B. At the focus of B is a small Faraday cylinder, F’, connected to an 
electrometer, E. A small impulse is derived from the wire, W, by 
electrostatic induction at D, and travels down to B., If the various 
distances and wire lengths are properly adjusted, so that the X rays 
arrive at B at the same moment as the derived impulse, electrons are 
liberated at B by the rays, and guided by the impulse into the cylin- 
der, F, and thence to the electrometer. If now the distance of the 
X-ray bulb from B is altered, say, by an increase of 10 cm., the wire 
from D to B has to be lengthened by 10 cm. Thus, according to 
Marx, the X rays travel with the same velocity as the impulse in the 
wire, and therefore with the velocity of light. 
