﻿by Moving Electrified Particles. 451 



energy greater than W from the atom by the primary rays. 

 The photographs taken by Mr. C. T. R. Wilson o£ the drops 

 of water deposited on the ions produced by cathode rays 

 show that this secondary ionization is not considerable. 



For cathode rays with a velocity of 4*7 XlO 9 cm./^ec. 

 Glasson found that 1*5 pairs of ions were made per cm. of 

 path in air at the pressure of 1 mm. of mercury, so that at 

 atmospheric pressure 1140 pairs of ions would be produced 

 per cm. 



To compare this with the value given by our formula we 

 require to know the value of W, the energy required to ionize 

 an atom of the gas through which the ray is passing. This 

 is a quantity to which very different values have been 

 assigned by different observers, ranging in the case of air 

 from that due to a fall of the atomic charge through 2 volts, 

 found by H. A. Wilson, to that due to a fall through 175, 

 the value given by Rutherford. The production of secondary 

 positive rays by uncharged hydrogen molecules moving with 

 a velocity not greater than 1*4 x 10 8 cm. /sec. past corpuscles 

 at rest, shows that the energy required to ionize an atom of 

 hydrogen must be less than that possessed by a corpuscle 

 moving at a speed of 1*4 x 10 8 cm./sec„ which is equivalent 

 to a fall through 5 volts. We shall take as the basis of our 

 calculations for air Townsend's estimate of 10 volts. 



Glasson's rays moving with a velocity of 4*7 x 10 9 cm. /sec. 

 had kinetic energy equivalent to a fall through 6000 volts: 



; T 6000 10 i^m to 



hence it 1 = ■ ,j~-- e, W = ^7^ e, and ^ = 4 ? 7 x 10 ~ 10 . 



7177-^1140 x WT, 



if n is the number of corpuscles per c.c. of air at atmospheric 

 pressure. This gives n — 10'7 x 10 20 , the number of mole- 

 cules in the air =2*7 x 10 19 . Hence on an average there are 

 about 40 corpuscles to the molecule. If the number of 

 corpuscles in the atom were equal to the atomic weight, this 

 number would have been about 30: hence, unless the value 

 of W is very far out, we may conclude that the number of 

 corpuscles in an atom is not greater than 2 or 3 times the 

 atomic weight. 



We shall proceed to the consideration of ionization by the 

 ft particles from radium. Eve (Phil. Mag. Oct. 1911) found 

 that the ft rays from radium produced 48 ions per cm. of 

 path in air at atmospheric pressure; the velocity of these 

 rays is not given, but we can test the formula by calculating 

 from it what the velocity of the rays should be. Since 

 Grlasson's rays give 1 110 ions per em., and Eve's only 48, 



2 II 2 



