the Tons in Gases at Low Pressures. 557 



nature as those which carry it in the cathode rays and in the 

 electrification arising from the action of ultra-violet light. 



The unipolar positive leak wh'ch occurs from an incandes- 

 cent platinum wire in air or oxygen, and in which the moving- 

 bodies are positively electrified, was found not to be affected 

 by a magnetic fieM of the order of that used in the experi- 

 ments on the negative leak. This had already been observed 

 by Elster and Geitel (Wied. Ann. xxxviii. p. 27). 



On the theory of the effect given in this paper, the absence 

 of magnetic effect on the positively charged earners indicates 

 that e/m is much smaller or m/e much larger for the positive ions 

 than it is for the negative. I am engaged with some experi- 

 ments on the effect of the magnetic field on the convection of 

 electricity by positive ions, using very strong magnetic fields 

 produced by a powerful electromagnet kindly lent to me by 

 Professor Ewing. From the results I have already got, it is 

 clear that m/e for the positive ions produced by an incandes- 

 cent wire must be at least 1000 times the value for the 

 negative ions, and this is only an inferior limit. 



The positive and negative ions produced by incandescent 

 solids show the same disproportion of mass as is shown by the 

 positive and negative ions in a vacuum-tube at low pressures. 



W. Wien (Wied. Ann. lxv. p. 440) and Ewers (Wied. 

 Ann. lxix. p. 187) have measured the ratio of m/e for the 

 positive ions in such a tube, and found that it is of the same 

 order as the value of m/e in ordinary electrolysis ; Ewers 

 has shown that it depends on the metal of which the cathode 

 is made. Thus the carriers of positive electricity at low 

 pressures seem to be ordinary molecules, while the carriers of 

 negative electricity are very much smaller. 



Measurement of the Charge on the Ion produced by the 

 Action of Utra- Violet Light on a Zinc Plate. 



This charge was determined by the method used by me to 

 measure the charge on the ions produced by the action of 

 Rontgen rays on a gas (Phil. Mag. Dec. 1898); for the 

 details of the method I shall refer to my former paper, and 

 here give only an outline of the principle on which the method 

 is based. Mr. C. T. P. Wilson (Phil. Trans. 1898) dis- 

 covered that the ions produced by ultra-violet light act like 

 those produced by Rontgen rays, in forming miclei around 

 which water will condense from dust-free air when the super- 

 saturation exceeds a certain definite value. 



Suppose, then, we wish to find the number of ions pro- 

 duced by ultra-violet light in a cubic centimetre of air. We 

 cool the air by a sudden expansion until the snpersatnrntion 



