214 Prof. Thomson, Some experiments on Canal- Strahlen. 



enormously greater than the velocity of the canal-strahlen which 

 we have seen possess the power of ionization. The most probable 

 explanation of this difference seems to me to be, that when the 

 velocity of the a particles sinks below a certain value, it will hold 

 fast to a corpuscle against which it may come in collision, and 

 thus have its positive charge neutralized and become an un- 

 charged atom. If v is the relative velocity of the a particle and 

 a corpuscle at collision, when r is the distance between the 

 corpuscle and the particle, m the mass, e the charge on the 

 corpuscle, we can easily prove that the corpuscle will stick to the 

 particle if 



„ 2e 2 

 mv- < — , 

 r 



while the particle will escape if 



mv 2 > — . 

 r 



To find the smallest velocity to enable an a particle to escape, 

 let us give to r the conventional value for the radius of an atom, 

 10 -8 cm./sec, we have also in electrostatic measure 

 e/m = 3 x 10 17 and e = 3'5 x lO" 10 , 

 hence v = 1*4 x 10 8 cm./sec. will be the smallest velocity an a 

 particle can possess if it is to retain a positive charge. We need 

 not attach importance to the fact that the limiting value of v 

 found by Rutherford is considerably greater than this, for the 

 value assumed for r has no claim to accuracy. In the case of 

 the a particles the proportion of corpuscles to positive ions is 

 very much greater than in that of the canal-strahlen, so that 

 neutralization is more likely to occur in the former than in the 

 latter case. 



The canal-strahlen disintegrate a metal plate against which 

 they strike, for after a long continued bombardment of the 

 metal plate the walls of the tube in the neighbourhood of the 

 plate are found to be covered with a deposit of the metal. 



When the canal-strahlen impinge on a salt of sodium, the salt 

 gives out yellow light in which the D line is very bright, it is 

 remarkable that this line is not given out when canal-strahlen 

 impinge on the metal sodium itself, a mirror of sodium with 

 specks of oxide on it presents a beautiful appearance when 

 struck by these rays, the spots of oxide shine out brightly with a 

 greenish yellow light, while the surface of the metal itself seems 

 quite dull if no external light falls upon it. 



I have much pleasure in thanking my assistant, Mr Everett, 

 for the help he has given me with these experiments. 



