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



though marked, was not so great as this,, pointing to the con- 

 clusion that all the positive ions had not been caught by the 

 larger expansion. . 



It is obvious that since the moisture begins to deposit 

 first on the negative ions, unless the rate of expansion is very 

 rapid, the drops formed in the earlier stages of the expansion 

 round the negative ions will have time to grow and form 

 convenient nuclei for further condensation, so that as the 

 expansion increases, the tendency will be for the moisture to 

 deposit on the drops already formed rather than to form new 

 drops round the positive ions. r J hus with slow expansions we 

 should expect the number of drops formed, to be more nearly 

 equal to the number of negative ions than to the sum of the 

 positive and negative ions. 



The results of the present experiments show that this was 

 the case in the expansions used in the earlier experiments. 

 The value of n determined in those experiments was but little 

 greater than the number of negative ions; as ne is the quantity 

 determined by the electrical experiments this caused the value 

 of e to be nearly twice its real value. 



The results of Mr. C. T. E. Wilson's experiments give us 

 the means of testing whether the expansion is rapid enough 

 to catch the positive as well as the negative ions, for if we 

 determine the number of drops in the cloud when the ex- 

 pansion is less than 1*31, and again for expansions considerablv 

 greater than this value, and find that the latter number is 

 twice the former, we may feel sure that the large expansion 

 has been rapid enough to catch the positive ions. This test 

 has been applied throughout the experiments described in 

 this paper. 



The apparatus for producing the cloud was of the kind 

 used by Mr. Wilson . The expansion is produced by the motion 

 of a light glass piston P which slides freely up and down the 

 larger tube C, the lower end of the piston is always below 

 the surface of a layer of water at the bottom of the tube. The 

 inside of the piston is put in connexion with a vessel E by 

 means of tube F, which passes up through the layer of water. 

 H is a large vessel exhausted to as low a pressure as possible; 

 H and E are connected by the tube G, the end of this tube 

 in E is closed by an indiarubber pad which can be quickly 

 removed by means of the rod R; when this is done (the 

 piston being at the top of its range) the pressure of the "air 

 drives the piston down, and the air in C and the vessels con- 

 nected with it expand. The amount of this expansion can 

 be adjusted by altering the height to which the piston is 

 raised before the expansion begins. 



