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 j 
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 1 
be due to a leak through the gas and not along the insulating j 
support. 
Such an electroscope was placed in a vessel which was f 
filled with a mixture of equal volumes of hydrogen and chlorine I 
prepared by electrolysis. The arrangements for the preparation j; 
of the gases and the measurement of the amount of hydrochloric 51 
acid gas formed being the same as those used by Bunsen and k 
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. R. Wilson, Proc. Camb. Phil. Soc., Vol. xi. p. 32. 
