XI] AND OF ORDINARY MATERIALS 375 
Both Geitel and Wilson found that the leakage of the insulated 
system in dust-free air was the same for a positive as for a negative 
charge, and was independent of the potential over a considerable 
range. The leakage was the same in the dark as in diffuse 
daylight. ‘The independence of leakage of the potential is strong 
evidence that the loss of charge is due to a constant ionization of 
the air. When the electric field acting on the gas exceeds a 
certain value all the ions are carried to the electrodes before re- 
combination occurs. A saturation current is reached, and it will 
be independent of further increase of the electric field, provided, 
of course, a potential sufficiently high to cause a spark to pass is 
not applied. 
C. T. R. Wilson has recently devised a striking experiment to 
show the presence of ions in dust-free air which is not exposed to 
any external ionizing agency. Two large metal plates are placed 
in a glass vessel connected to an expansion apparatus similar to 
that described in section 34. On expanding the air the presence 
of the ions is shown by the appearance of a slight cloud between 
the plates. These condensation nuclei carry an electric charge 
and are apparently similar in all respects to the ions produced 
in gases by X rays or by the rays from active substances. 
Wilson found that the loss of charge of the insulated system 
was independent of the locality. The rate of discharge was un- 
altered when the apparatus was placed in a deep tunnel, so that 
it did not appear that the loss of charge was due to an external 
radiation. From experiments already described, however (section 
215), it is probable that about 30 per cent. of the rate of discharge 
observed was due to a very penetrating radiation. This experiment 
of Wilson’s indicates that the intensity of the penetrating radiation 
was the same in the tunnel as at the earth’s surface. Wilson 
found that the ionization of the air was about the same in a brass 
vessel as in one of glass, and came to the conclusion that the 
air was spontaneously ionized. 
Using a brass vessel of volume about 471 c.c., Wilson de- 
termined the number of ions that must be produced in air 
per unit volume per second, in order to account for the loss of 
charge of the imsulated system. The leakage system was found 
to have a capacity of about 1:1 electrostatic units, and lost its 
