212 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
But it is to be remembered that the electrons which are liberated 
by X rays have an initial velocity averaging about 510° per sec., 
i. e., a speed due to thousands of volts, and are scattered in all direc- 
tions from the surface on which the rays fall. Neither the weak im- 
pulse applied to B by the wave coming along the wire, DB, nor the 
peculiar form of the surface, B, could have any sensible effect in the 
way of guiding these fast-moving electrons into the cylinder, F. Only 
slow-moving electrons or 8 rays could be guided by such means. It is 
no doubt true that X rays do liberate a certain number of 6 rays, 
but it is clear that the experiment of Marx is quite consistent with the 
hypothesis that the X rays are complex, and consist in part of ether 
pulses traveling with the velocity of light, and producing 6 rays, 
and in part of material particles, or pairs, traveling at a speed as yet 
undetermined, and exciting high-speed cathode rays. It would be 
reasonable to expect that a stream of pairs should be accompanied 
by ether pulses which had their origin at the time and place where the 
pairs broke away. 
It is possible that the example of the a particle shows that a pair 
can not possess a velocity greater than 10°, since at a higher speed 
it would be stripped of an electron, and become an a particle. J. J. 
Thomson has suggested that at this critical speed the «a particle be- 
comes electrically neutralized by the attachment of an electron. Pre- 
sumably such a pair would then go on as a y ray. No such conse- 
quence has been observed; and on the present hypothesis it would be 
better to suppose that the a particle ends its career by being taken up 
by an atom, as Rutherford has suggested. There is no reason to sup- 
pose the y ray or X ray to possess any great speed, so as to give it 
enough penetrating power. The latter might depend rather on the 
limitation of the field of the pair; and a sufficient range for the veloc- 
ity can be found between the minimum speed of the a particle and the 
maximum speed necessary for penetration, which appears to be about 
108 for a charged particle, but may be less for one without charge. A 
moderate speed would account for the reflection or scattering of the 
X ray, and would indeed be necessary for this purpose. 
To sum up, it is clear that a stream of X rays contains some ether 
pulses, but it is not easy to explain all the properties of X rays on the 
ether-pulse theory. The explanations are easier if the rays are sup- 
posed to consist mainly of neutral pairs; and the existence of such 
pairs is not improbable @ priorz. 
[Added July 18, 1907.] 
Since this was written several important papers have appeared, 
with which the outlined theory seems to me to be in harmony. 
I have supposed it possible for positive electrons to be detached 
from atoms of matter in the X-ray tube and to be sent out in com- 
pany with negative electrons, one of each going to the formation of 
