﻿Ionization by Positive Bays. 793 



analogy between the maximum with cathode rays at 5 volts 

 followed by a fall, and the maximum with positive rays at 

 38,000 volts also followed by a fall, is not wholly false. 

 The cause of both falls may be the same, namely the 

 beginning of the stage at which the rays penetrate the 

 surface of the plate. 



11. It is suggested then that the fall of P for values of Y 

 greater than 38,000 volts represents the be "inning of the 

 stage at which the positive rays acquire one of the properties 

 of a-rays — the power of penetrating atoms. And if this be 

 so_, we should expect that it would be followed shortly by the 

 acquirement by the rays of the other property of a-rays, a 

 great ionizing power ; we should expect that P after falling 

 to a minimum should increase rapidly to a value charac- 

 teristic of a-rays, and the form of the curve suggests, 

 perhaps, that this rise should occur for some value of V less 

 than 100,000 volts. It may, then, be possibly capable of 

 observation. 



If this view is correct, we should expect R as well as P 

 to decrease after the maximum of P is attained. Unfortu- 

 nately, as has been said, no evidence on this point is available. 

 It is not to be expected that a similar fall in ionizing 

 power would be found if the observations were made in a 

 gas in which the rays are completely absorbed. For if, in 

 the process of absorption in a gas, the speed of the rays is 

 reduced to zero, it is almost impossible that faster rays should 

 possess a smaller total ionizing power than slower rays : they 

 could only possess (as in the case of a-rays) a smaller ionizing 

 power over some portion of their range. 



It remains to consider whether the view suggested is 

 supported or refuted by any known facts. A definite 

 penetration of atoms by the positive particles of canal-rays 

 does not seem to have been observed. Goldsmith * has 

 described observations which appear to show that such 

 particles can make their ways through sheets of mica about 

 0*005 mm. thick, even when their energy is as low as 

 10,000 volts. But, since he could find no trace of rays after 

 penetration, even when the initial energy was 35,000 volts, 

 it is probable that such particles make their way between 

 the atoms rather than penetrate through them, as do a-rays. 

 On this side, then, there is no evidence that their penetration 

 does not begin at about 38,000 volts. 



On the other side, it is almost impossible to fix the least 

 energy which confers on particles the properties of an a-rav. 

 The least velocity of such rays which has been measured is 

 * A. N. Goldsmith, Phys. Rev. ii. p. 16 (1913). 



