438 Mil. C. T. E. WILSON OX THE COXDEXSATIOX NUCLEI PEODUCED IN 
intensity; the number of drops produced on expansion, indicating as it does the 
number of cariiei'S which at that instant are on their way across between the plates, 
Avill therefore be inversely projDortional to the difference of potential. The ^^henomenon 
is, in fact, closely related to the fact first noticed by Stoletow,"^ that the currents 
produced by the action of ultra-violet light on negatively electrified surfaces a23proach 
a saturation A'alue as the electromotiAm force is increased. 
The fact that the source of the rays is discontinuous must also not be forgotten, 
for Avith strong fields all the ions produced by the action of one spark may haA’e time 
to traAml across to the other plate, under the influence of the electric field, before the 
next spark takes place. 
The fact that the carriers in hydrogen, as RuTHEEFOEDt has shoAAm, traAml 
seAmral times as fast as in air, explains Avhy the phenomenon under consideration Avas 
only observed in the former gas. With greater differences of potential, or a smaller 
distance between the plates, it Avould no doubt be observed in air also. 
The fact that the nuclei produced by the action of ultra-Anolet light on a 
negatively electrified zinc plate Avhether in air or hydrogen, require just the same 
degree of supersaturation in order that they may act as centres of condensation as 
those produced by Rontgen rays or Uranium rays, is strong eAudence that the carriers 
of the electricity in all these cases are of the same nature. Rutheefoed;]; has affeady 
proved this in quite a different Avay by his measurements of the Amlocity with which 
the carriers move in an electric field. 
The very considerable degree of supersaturation necessary to make condensation 
take place on these nuclei is of itself sufficient to proA^e that the particles Avhich carry 
off the iiegatiA-m charge from the zinc-plate are not of the nature of dust particles, but 
on the contrary must be of almost molecular dimensions (vide ‘ Phil. Trans.,’ loc. cit., 
p. 305). 
The conclusion arrived at is therefore opposed to that AAdiich Lexaed and Wolff 
clreAv from the results of their steam-jet experiments {loc. cit.), that the escape of 
negative electricity from a zinc plate exposed to ultra-violet light is due to the escape 
of particles arising from the disintegration of the metal. As Avas pointed out by 
R. v. Helmholtz and Richaez,§ the steam jet is incompetent to distinguish between 
dust particles and the “ ions,” to AAdiich the latter obserAmrs attribute most of the 
condensation phenomena studied by them. 
* Stoletoaa’’, ‘ Comptes Eeudiis,’ 106, p. 1149, 1888. 
t Eutherford, ‘Camb. Phil. Soc. Proc.,’ vol. 9, p. 401, 1898. 
I Eutherford, ‘ Camb. Phil. Soc. Proc.,’ loc. cit. 
§ E. V. Helmholtz and Eicharz, ‘ Wied. Ann.,’ 40, p. 161, 1890. 
