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46 IONIZATION THEORY OF GASES [CH. 
35. Difference between the positive and negative ions. 
In the course of experiments to determine the charge carried by 
an ion, J. J. Thomson! observed that the cloud formed under the 
influence of X rays increased in density when the expansion was 
about 1°31 and suggested in explanation that the positive and 
negative ions had different condensation points. 
This difference in behaviour of the positive and negative ions 
was Investigated in detail by C. T. R. Wilson? in the following way. 
X rays were made to pass in a narrow beam on either side of a 
plate AB (Fig. 7) dividing the condensation vessel into two equal 

Fig. 7. 
parts. The opposite poles of a battery of cells were connected 
with two parallel plates C and D, placed symmetrically with regard 
to A. The middle point of the battery and the plate A were con- 
nected with earth. Ifthe plate C is positively charged, the ions in 
the space C'A at a short distance from A are all negative in sign. 
Those to the right are all positive. It was found that condensation 
occurred only for the negative ions in AC when = = 1:25 but did 
1 
pe Dias Ob 
not occur in AD for the positive ions until —= 1:31. 
1 
1 Phil. Mag. p. 528, Dec. 1898. 
2 Phil. Trans. 193, p. 289, 1899. 
