198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
that each primary electron set free fewer secondary electrons, but 
that each of the latter set free fewer tertiaries, and so on, so that 
the ionization would fall at a far greater rate than the pressure as 
soon as the free path of the electrons became comparable with the 
dimensions of the chamber. And, again, the @ rays differ only in 
speed from cathode rays, which produce quantities of slow-speed 
electrons, even where their own velocity is great. 
For these reasons I think it must be concluded that the £ particle 
(and any high-speed secondary) produces slow-speed electrons along 
its path, in very much the same way as the a particle does, though 
not in such great numbers. The high-speed secondary rays, studied 
by McClelland, Allen, and others, are but few in number compared 
to the slow-speed electrons, though their greater energy puts them 
more in evidence. McClelland concludes from his experiment that 
the B rays do not produce any slow-speed electrons, when they strike 
a metal surface, which are comparable in number with the electrons 
displaced in the gas through which they have passed. This is quite 
consistent with what has been said above. There must be a few, 
but the number to be expected is quite small, for the B electrons dive 
so deep into the metal which they strike, and ionize so few of the 
molecules through which they pass, that very few of the slow-speed, 
highly absorbable electrons can be discharged from the surface of 
the plate. Even in the case of the « particle these electrons are not 
readily observed; in the case of the 8 particle the difficulty must be 
much greater. 
As regards X rays, we have no such accurate measurements of the 
velocities of the electrons which are ejected from the molecules of a 
gas traversed by the rays, as we have in the case of the cathode rays, 
so far as I am aware. But a very large amount of labor has been 
spent on the investigation of the secondary radiation caused by the 
X rays, from which we may gather much indirect evidence on the 
point. Perrin (Ann. Chim. Phys., XI, p. 496, 1897) has shown that 
the rate of production of ions per ce. by rays of given intensity is 
proportional to the pressure of the gas. Again, we know from the 
investigations of Curie and Sagnac, Townsend and Barkla that 
metals struck by X rays return a secondary radiation, which, in the 
case of the low atomic weights, may be considered to consist princi- 
pally of scattered primary radiation, and in the case of the high 
atomic weights to contain both X rays more absorbable than the pri- 
mary and cathode rays. Dorn has shown that the latter have speeds 
averaging about 510° cm., so that they must produce considerable 
ionization, consisting of 8 rays, in the few millimeters of air close to 
the metal. The free path of electrons having this speed is about one 
millimeter in air at atmospheric pressure. Since the X rays do not 
