208 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
lighter the atom that would do it. (See J. J. Thomson on Barkla’s 
researches, “ Electrician,” April 5, 1907.) 
In order to explain these known effects on the ether-pulse theory 
it is necessary to suppose that in light atoms the corpuscles are not 
appreciably acted on by forces due to other corpuscles, but that in 
heavy atoms there is a strong influence of this kind. In the former 
case the thickness of the secondary pulse is the same as that of the 
primary; in the latter it is not. It is also necessary to suppose that 
when the atom is heavy enough to cause a modification of the primary 
radiation, it differs from a light atom in such a way that the pulse 
can cause cathode particles to be ejected at a speed due to thousands 
of volts, whereas this is impossible with ight atoms. 
Tf the cathode particles in the X-ray tube so affect the motion of 
an atom which they strike as to make it throw off a pair, then the 
plane of rotation of the pair will be the same as that of the atom 
~ from which it has come, and will contain the direction of the trans- 
latory motion of the pair. The pair will therefore be able to show 
polarization effects, and if such a pair falls upon a reflecting surface, 
it is not unreasonable to suppose that it is hable to be taken up only 
by an atom revolving in the same plane, and sometimes to be ejected 
again. Thus its subsequent rotation and translation will continue to 
take place in the one plane. The tertiary ray will therefore be 
strongest when it is in the same plane as the primary and secondary ; 
and this is Barkla’s polarization effect. 
If the X ray is an ether pulse, it is difficult to understand why the 
spreading pulse affects so few of the atoms passed over (“ Conduc- 
tion of Electricity through Gases,” pp. 294-297), why the high-speed 
secondary cathode rays are ejected with a velocity which is independ- 
ent of the intensity of the pulse, and why it should be able to exercise 
ionizing powers when its energy is distributed over so wide a surface 
as that of a sphere of say 10 or 20 feet radius. All these phenomena 
are more simply explained if we suppose the ray to be a neutral pair 
which has only a local action, i. e., can only affect the molecules on its 
path, which can penetrate to great distances in air, losing little speed 
as it goes, and which gives rise to a cathode ray when it is broken by 
impact. 
It seems to me that the material-nature hypothesis shows to advan- 
tage when we consider the secondary radiation of the X rays. The 
rays cause the emission of cathode rays whose speed averages about 
510°. (Dorn.) We have no experience of any ether wave caus- 
ing the emission of any but 8 rays, i. e., electrons with a speed of 
about 10%. It can hardly be said that differences in intensity of 
the ether pulse can account for this remarkable contrast, for the 
speed of the 8 rays caused by ultra-violet light has been shown by 
Lenard to be independent of the intensity of the light, and the ve- 
