(4ASES BY THE ACTION OF EONTC4EN BAYS AND OTHER AGENTS. 
449 
Difference of potential = 240 volts ; thickness of air layer = 1 ’4 centims. 
Gauge reading (in mil- 
lims.) = p. 
Result of 
expansion. 
No E.M.F. 
Zinc positive. 
Zinc negative. 
16.5 
Slight shower 
Slight shower 
Slight shower 
188 
Dense shower 
Dense shower 
Dense shower 
Thickness of air layer = '8 centims. 
189 
Dense shower 
Dense shower 
Dense shower 
217 
Dense fog 
Dense fog 
Dense fog 
Exactly similar results were obtained when the zinc was amalgamated, so that a 
larger number of nuclei might be produced. 
The action of zinc in producing nuclei is thus proved to be of quite a different 
nature to that of uranium oxide. It does not consist in the production of free ions 
throughout the volume of the air near it by the action of radiation like that from 
uranium and its compounds. 
It might be supposed, however, that the nuclei consisted of ions, not produced 
throughout the volume of the air, but having their origin at the surface of the zinc. 
They might in fact be a direct product of the oxidation of the zinc, the oxygen or water 
molecules being split up, half of the molecule combining wdth the zinc, the other 
part escaping into the surrounding gas as a free ion. One would expect the ions, 
according to this view, to be all charged with electricity of the same sign. There 
ought therefore to have been a difference in the number of cloud particles produced 
according as the zinc was made positive or negative. In fact one would expect, as 
was found to be the case when the zinc was exposed to ultra-violet light, an increase 
in the number of nuclei when the electromotive force was in one direction, that 
namely tending to move the ions from the zinc, and a diminution when the field was 
reversed. The absence of any difference whatever in the appearance of the fogs whether 
the zinc w'as the positive or negative terminal or was uncharged, shows that the 
nuclei do not consist of ions, produced either at the surface of the metal or through¬ 
out the volume of the air and in its neighbourhood. They are, like the nuclei 
produced by the action of ultra-violet light on moist air, uncharged. 
Ions are thus not the only nuclei requiring exjiansions between the limits 
= 1’25 and Vojv^ = 1’37, in order that condensation may take place upon them ; 
both weak ultra-violet light and certain metals produce such nuclei, which experiment 
shows to be unaffected by an electric field, that is, not to be ions. They have less 
definite properties as nuclei of condensation than the ions ; the minimum expansion 
VOL. CXCII.-A. 3 M 
