
50 IONIZATION THEORY OF GASES [CH. 
The moist gases were saturated with water vapour at a tem- 
perature of 15°C. 
It is seen that the negative ion in all cases diffuses faster than 
the positive. Theory shows that the coefficients of diffusion should 
be directly proportional to the velocities of the ions, so that this 
result is in agreement with the observations on the greater velocity 
of the negative ion. 
This difference in the rate of diffusion of the ions at once 
explains an interesting experimental result. If ionized gases are 
blown through a metal tube, the tube gaims a negative charge 
while the gas itself retains a positive charge. The number of 
positive and negative ions present in the gas is originally the same, 
but, in consequence of the more rapid diffusion of the negative ions, 
more of the negative ions than of the positive give up their charges 
to the tube. The tube consequently gains a negative charge and 
the gas a positive charge. 
38. A very important result can at once be deduced when the 
velocities and coefficients of diffusion of the ions are known. 
Townsend (loc. cit.) has shown that the equation of motion of the 
ions 1s expressed by the formula 
if 
K 
dp 
pu=— as +nXe, 
da 
where e is the charge on an ion, 
n =number of ions per c.c., 
p = their partial pressure, 
and w the velocity due to the electric force X in the direction of 
the axis of z. When a steady state is reached, 
dp nX eK 
—-=( and u= 
i Pp 
Let VV be the number of molecules in a cubic centimetre of 
gas at the pressure P and at the temperature 15°C., for which 

; : N 
the values of wu and K have been determined. Then = may be 
Je 
3 Sen : : , 
substituted for —, and, since P at atmospheric pressure 1s 10°, 
P 
