350 Prof. J. J. Thomson on the Charge of 



Thus we see that when the expansion is greater than 1*33, 

 the number of nuclei caught by the cloud does not depend 

 upon the amount of the expansion. "When this is between 1*33 

 and 1'29 this number diminishes, until when the expansion 

 is 1*29 the number caught is only half that at the higher 

 expansions. The number now remains independent of the 

 expansion until this falls below about 1*27; for expansions 

 smaller than this the number of nuclei caught falls off rapidly 

 with the expansion. We conclude from this that for ex- 

 pansions greater than 1*33 all the ions positive as well as 

 negative are caught by the cloud ; then when the expansion 

 is smaller than 1*33 and greater than 1*29 some, but not all, 

 of the positive ions and all the negative ions ; for expansions 

 from 1*29 to 1'27 all the negative but none of the positive 

 ions are caught in the cloud ; and that when the expansion 

 falls below 1*27 only a fraction of the negative ions are 

 caught. 



The values of the numbers given in the third column of 

 the preceding table were calculated by the following method: 

 let q be the volume of water deposited through the expansion 

 per unit volume of the expanded gas (this was calculated by 

 the method given in the earlier paper), m! the number of 

 drops per unit volume of the expanded gas, a the radius of 



\rrr 



one of these drops; then q — n ! — a?. 



o 



If v is the rate at which the drops fall, then 



qa. 



where //, is the coefficient of viscosity of the gas. In the case 

 of air fju=lS x 1(J- 4 and v is therefore equal to 121 x 10 4 a 2 ; 



hence 



k 



The number of drops reckoned per unit volume of the gas 

 before expansion is n! X expansion. The velocity of the drops 

 was determined by measuring the time taken 'by the top of 

 the cloud to fall through two consecutive centimetres. With 

 the drops used, these times were very approximately equal 

 to each other ; with clouds consisting of very fine drops 

 which fall slowly, the time taken to fall through the second 

 centimetre is often considerably greater than that for the 

 first, owing to the diminution produced by evaporation in the 

 size of the drops. 



To facilitate the measurement of the velocity the inside of 



