562 



Prof. J. J. Thomson on the Masses of 



This is the number in 1 cub. centim. of the expanded gas; 

 the number in 1 cub. centim. of the gas before expansion is 

 1'36 n'. To find n the number of ions we must subtract from 

 1*36 n' the number of drops which are formed when the 

 ultra-violet light does not fall on the plate. With an expansion 

 as large as 1*36, Mr. Wilson has shown that a few drops are 

 always formed in dust-free air, even when free from the in- 

 fluence of Rontgen rays or ultra-violet light. If V be the 

 velocity with which these drops formed in the absence of the 

 light fall, then the number of drops due to these nuclei is 



1-36x5000 

 3-14 VI ' 

 Subtracting this from 1*36 n f , we find 



» = 2-07xl0 3 (4 



\ri VI- J' 



In making this correction we have assumed that the clouds 

 form round these nuclei even when the negative ions due to 

 the ultra-violet light are present. If the cloud formed more 

 readily about the negative ions than about the nuclei, the ions 

 would rob the nuclei of their water, and we should not need 

 the correction. The following table gives the result of some 

 experiments ; in making the observation on the cloud the 

 same potential-difference between the plate and the water 

 was used as when observing the value of d/~D : u was deter- 

 mined by Prof. Rutherford as 1*5 X 3 X 10 2 , and A was 

 7r(l'6) 2 throughout the experiments. 



b. 



C. 



dIB. 



v. 



V. 



exKF. 



1-2 



62 



•0017 



•13 



•3 



7-9 



1-2 



62 



•0019 



11 



•3. 



73 



•9 



50 



•0012 



•14 



•3 



53 



1-2 



65 



•0035 



•08 



•3 



7-3 



1-2 



50 



•0018 



11 



•3 



6 



1-2 



40 



•0018 



•14 



•3 



7 



The mean value of e is 6*8 x 10~ 10 . The values differ a 

 good deal, but we could not expect a very close agreement 

 unless we could procure an absolutely constant source of ultra- 

 violet light, ;is these experiments are very dependent on the 

 constancy of the light; since the electrical part of the experi- 

 ment measures the average intensity of the light over 30 



