90 Prof. Thomson ; Are any free charged ions produced 



On the question as to whether or not there are any free charged 

 ions produced during the combination of hydrogen and chlorine ; 

 and on the effect produced on the rate of the combination by the 

 presence of such ions. By J. J. Thomson, M.A., F.R.S. 



[Received 16 January 1901.] 



The experiments described below were made to see whether or 

 not free ions charged with electricity are produced when hydrogen 

 and chlorine are combining under the influence of light. 



The electrical conductivity of a gas affords a very delicate test 

 for the presence of these ions, as the rate of leak of electricity 

 from a gold-leaf electroscope will be affected to an appreciable 

 extent by the presence of a few thousand ions per cubic centi- 

 metre ; and as the number of molecules of a gas at atmospheric 

 pressure in the same volume is about 10 20 , it will be evident 

 that the electrical test is one of exceptional delicacy. 



The electrical conductivity of the mixture of hydrogen and 

 chlorine was measured by observing the rate at which the 

 separation of the leaves of a gold-leaf electroscope immersed in 

 the gas diminished ; (in some experiments the leaves of the 

 electroscope were made of platinum-foil). Special care was 

 taken with the insulation of the electroscope, the same arrange- 

 ment as that used by Mr C. T. R. Wilson 1 was employed ; in this 

 form of electroscope the leaves are fastened to an ebonite rod 

 which passes through the sides of the vessel containing the mixed 

 gases, a metal ring is placed round this rod between the gold- 

 leaves and the junction of the rod with the vessel, the gold- 

 leaves are charged up to a high potential by means of a battery 

 of small storage cells, and the ring is kept in connection with this 

 battery so that it is maintained permanently at the same potential 

 as the original potential of the leaves ; it is evident that with this 

 arrangement any diminution in the divergence of the leaves must 

 be due to a leak through the gas and not along the insulating 

 support. 



Such an electroscope was placed in a vessel which was 

 filled with a mixture of equal volumes of hydrogen and chlorine 

 prepared by electrolysis. The arrangements for the preparation 

 of the gases and the measurement of the amount of hydrochloric 

 acid gas formed being the same as those used by Bunsen and 

 Roscoe in their researches on the combination of these gases. 



The combination of these gases may be divided into two 

 stages, for it has been shown by Draper and later by Pringsheim 



1 C. T. E. Wilson, Proc. Camb. Phil. Soc, Vol. xi. p. 32. 



