Ionization by Charged Particles. 815 



•experiment in respect of the ionizing power of very slow 

 yS rays, unless allowance is made for the smaller power of 

 snch rays to penetrate within the atom. It is very doubtful 

 whether any precise physical significance can be attributed 

 to the conception of " penetration within the atom " ; the 

 influence exerted by the forces determining atomic structure 

 is probably represented better in the work of Bohr (14), 

 who considers that the electrons are restrained by elastic 

 forces and capable of vibrations of definite frequency. Bohr's 

 analysis is inapplicable to cases where the disturbance caused 

 by the primary rays is so great as to give rise to ionization, 

 and it appears impossible to predict anything definite con- 

 cerning the velocities with which such electrons would be 

 liberated by rays so slow that the elastic forces are of 

 importance. But Bohr's view, as well as Lenard's, might 

 -explain the existence of a very powerful " reflexion " of rays 

 too slow to ionize, for the elastic forces restraining the 

 •electrons would cause them to behave towards such slow 

 rays as if they were held rigidly ; at an encounter with such 

 rigid electrons a primary ray would be powerfully deflected, 

 but would suffer little loss of energy. It is only when the 

 electron is free that the deflexion of a ray with which it 

 reacts increases with the energy which it takes from that 

 ray, as shown by (1) and (2). 



These two equations also show that a primary ray will 

 not be deflected through a right angle, and so appear as a 

 "" reflected electron'" in consequence of a reaction with a free 

 electron, unless it gives to that electron half its energy. By 

 this consideration we might explain the absence of electrons 

 leaving the incidence side of a plate struck by primary rays 

 with a velocity of 80 volts, and having a velocity between 

 80 and 30 volts. However, the absence of 8 rays with 

 velocities greater than 30 volts, whatever the velocity of the 

 primary, is more probably to be attributed to the fact that 

 such rays can produce further ionization, and so tend to 

 lose energy rapidly before they emerge. There may be 

 some 8 rays faster than 30 volts, but they may be difficult 

 to detect because they cause the emission of fresh 8 rays from 

 the electrode at which they are received. Indeed, Bumstead's 

 secondary /3 rays excited by a rays are probably such very 

 fast 8 rays liberated by the direct action of the a rays on the 

 electrons of the atoms through which they pass. Since an 

 a. ray with a velocity of 2 x 10 9 cm./sec. behaves, according 

 to Thomson's theory, like a j3 ray with twice that speed, 

 (1) shows that such an a ray could liberate 8 rays with an 

 -energy of 4500 volts, a value higher than that attributed by 



