Power of Positive Pays to produce Phosphorescence. 617 



on the mass m of the corpuscle, and on the distance a between 

 the two bodies, which may be put equal to the radius of the 

 particle. The condition, that the particle should leave the 

 corpuscle, is that ^mv 2 should be greater than Ee/a. I£ this 

 condition is not fulfilled, the corpuscle becomes a satellite of 

 the particle and makes it electrically neutral. If we put 



1=5-1 x 10 17 , E = £ = 10- 10 , « = 10 -8 , 

 m 



we find for the limiting value v = l'9 x 10 8 cm. sec -1 . If the 

 velocity of the oc particles falls below this value, it is possible 

 for a neutralization to take place. 



In his recent researches on the positive rays, Prof. J. J. 

 Thomson has found the velocity of the positive particles to 

 be 1*8 X 10 8 cm. sec -1 , measured at some distance from the 

 cathode. The velocity of the particles, when they cease to 

 produce phosphorescence, must be, however, somewhat smaller 

 than this value on account of the passage of the particles 

 through the gas. The value of a is the conventional one and 

 is not deduced from any property of the canal rays. With 

 regard to this circumstance, we may consider the agreement 

 between the theory and the experiment as well established. 



According to the measurements of Rutherford, the velocitv 

 of the a. particles of radium, just before they lose their power 

 of producing phosphorescence, is 1*1 x 10 9 cm. sec -1 , a velocity 

 much greater than the velocity of the canal rays, when start- 

 ing from the cathode, and the velocity of these rays is still 

 smaller when the rays cease to affect the phosphorescent 

 screen. This remarkable difference in the velocity of the 

 positive ra} r s of the two different sources leads to the sugges- 

 tion that the kinetic energy possessed by the positive particles, 

 which impinge on unit surface per unit time, must exceed a 

 certain value if the phosphorescence on the willemite screen 

 is to be produced. In the case of the a particles of radium, 

 this amount of kinetic energy is represented by a small 

 number of particles moving with great velocity, but in the 

 case of the positive rays the same amount of kinetic energy 

 is given by a much larger number of particles moving with 

 smaller velocity ; the energy required for producing phos- 

 phorescence in the latter case is stored up between the 

 successive impacts of the particles. 



In conclusion, I have great pleasure in thanking Professor 

 J. J. Thomson for his kind interest and his helpful sugges- 

 tions in this investigation, which was carried out in the 

 Cavendish Laboratory- 

 Cambridge, 10th August, 1907. 



