m1] IONIZATION THEORY OF GASES 45 
greater the intensity of the rays. This point at which condensa- 
tion begins is very marked, and a slight variation of the amount of 
expansion causes either a dense cloud or no cloud at all. 
It now remains to be shown that the formation of a cloud by 
the action of the rays is due to the productions of ions in the 
gas. If the expansion vessel is provided with two parallel plates 
between which an electric field can be applied, it is seen that the 
number of drops, formed by the expansion with the rays acting, 
decreases with increase of the electric field. The stronger the 
field the smaller the number of drops formed. This result is to be 
expected if the ions are the centres of condensation ; for in a strong 
electric field the ions are at once carried to the electrodes, and thus 
disappear from the gas. If no electric field is acting, a cloud can 
be produced some time after the rays have been cut off; but if a 
strong electric field is applied, under the same conditions, no cloud 
is formed. This is in agreement with experiments showing the 
time required for the ions to disappear by recombination. In 
addition it can be shown that each one of the fine drops carries an 
electric charge and can be made to move in a strong uniform 
electric field. 
The small number of drops produced without the action of the 
rays when ~ > 1-25 is due to a very slight natural ionization of 
1 
the gas. That this ionization exists has been clearly shown by 
electrical methods (section 218). 
The evidence is thus complete that the ions themselves serve 
as centres for the condensation of water around them. These ex- 
periments show conclusively that the passage of electricity through 
a gas is due to the production of charged ions distributed through- 
out the volume of the gas, and verify in a remarkable way the 
hypothesis of the discontinuous structure of the electric charges 
carried by matter. 
This property of the ions of acting as nuclei of condensation 
gives a very delicate method of detecting the presence of ions in 
the gas. If only an ion or two is present per c.c., their presence 
after expansion is at once observed by the drops formed. In this 
way the ionization due to a small quantity of uranium held a yard 
away from the condensation vessel is at once made manifest. 
