produced in Air by Ultra-Violet Light. 



489 



there is not much difference between the mobilities of the ions 

 produced by X-rays and those produced by ultra-violet light. 

 The figures at the end of the tables are not so reliable as those 

 at the beginning, for example the F corresponding to — 18 volts 

 in Table I is "073 and this was calculated from an electroscope 

 reading of '8 of a division, which from its small size is liable to 

 considerable error. 



Absolute agreement between the two sets of curves could only 

 be expected if the ions were distributed in the stream of air in 

 precisely the same way. It was thought, however, that considera- 

 tions of this sort could not wholly explain the differences between 

 the results given above and that a better agreement was possible. 



As nearly all the negative ions in the experiment may be put 

 down to surface ionisation, it was not necessary to investigate 

 their mobilities, for Rutherford* has shown that surface ions are 

 identical with negative ions produced by X-rays, etc. The results 

 for the positive and the negative ions produced by ultra-violet 

 light and the positive ions only produced by X-rays are given in 

 Tables III and IV. 



During the short time occupied in taking a reading, the 

 velocity of the air did not alter appreciably, but it was difficult 

 to adjust the velocity each time to exactly the same value. A 

 small correction was therefore made in the figures of Tables III 

 and IV. The pressure drop across the cotton-wool plug E was 

 about 150 cms., and it was assumed that the variations in the 

 velocity of the air were proportional to the small deviations from 

 this value and that the quantity of electricity F was increased in 

 the same proportion. Therefore, to obtain the value of F for a 

 standard velocity corresponding to a pressure drop of 150 cms. 

 the value was diminished in proportion to the excess of the 

 pressure over 150 cms. and conversely. This correction, which 

 was never more than about 5 % o^ 6 7o brought the results into 

 better agreement. 



Table III. Ionisation hy Ultra-Violet Light. 



Butherford, Proc. Camb. Phil. Soc. ix. p. 401, 1898. 



