366 Experiments on Induced-Radioactivity in Air. 



at which the conductivity dies away also shows that the drops 

 must settle down exceedingly slowly, so slowly that they take 

 some days to fall through 1 metre ; from this we may con- 

 clude that the diameter of the drop cannot greatly exceed 

 10~ 5 cm. If each little drop gets surrounded by a layer of 

 positive electricity then, just as in the case of the wire, the 

 drop might emit cathode-rays which would ionize the air in 

 its immediate neighbourhood ; thus each little drop would 

 act as a centre of ionization, and thus make the air a con- 

 ductor. The formation of a layer of positive electricity 

 outside the drop is what we should expect if any chemical 

 combination went on between the w r ater of the drop and the 

 oxygen of the air leading to the formation of such a com- 

 pound as H 2 2 , for, in forming this compound, the water 

 would combine with a negative oxygen ion and not with a 

 positive one ; thus from the layer of oxygen outside the drop 

 the water w T ould pick out the negative and leave the positive 

 ions, this would lead to the production of the coating of 

 positive electricity round the drop required to make it act as 

 an ionizing agent. 



The drops of water as well as acting as producers of ions 

 would also act as traps to catch ions moving through the 

 air in which they are suspended ; they thus tend to reduce 

 the conductivity, because when an ion gets attached to one of 

 these drops, it is as it were anchored to it, and only moves 

 with great difficulty ; in some cases the presence of drops 

 of water diminishes the conductivity of the gas instead of 

 increasing it : thus I found that squirting a steam-jet into 

 either of the tanks A or B materially diminished the satura- 

 tion-current through the tank. 



The ordinary polarization of the electrodes in the electrolysis 

 of liquids is usually explained by the existence of a layer of 

 electrification close to the surface of the electrode, thus the 

 polarized electrode resembles in this respect the electrified wire 

 and the small drop of water on the preceding theory. I there- 

 fore thought it of interest to see whether a polarized electrode, 

 when taken out of the electrolytic cell, would ionize the gas. 

 Two platinum plates or wires were immersed in a solution of 

 sulphuric acid of about the maximum conductivity, and a 

 current of from 1 to 5 amperes sent from one electrode to the 

 other for about an hour ; the electrodes were then taken out, 

 washed with distilled water, and dried with filter-paper ; they 

 were then placed in tank B and the saturation-current through 

 the tank when these were used as electrodes measured. It 

 was found that the one which had been used as the negative 

 electrode (i. e. the one against which the hydrogen was 



