44G MR. C. T. R. WILSOX OX THE COXDEXSATIOX XUCLEI PRODUCED IX 
the difference of potential just mentioned, no nuclei could be detected if the rays were 
cut off* before the expansion, even if this were effected as quickly as possible after the 
rays were stopped. On the other hand, wlien no electromotive force was applied, 
some of the nuclei lasted for at least 10 seconds after the rays were cut olF, a shower 
being even then obtained on expansion. 
There is thus a veiy marked difference in the l)ehaviour of the nuclei according as 
they are produced by Ihmtgen rays or ultra-violet rays, the nuclei produced by the 
latter being uninfluenced even by a comparatively strong field. The phenomena 
oliserved witli air exposed to Rdntgen rays a,re easily understood in the light of 
IIutherfoed’s experiments * on the velocity of the ions in air which has acquired 
conducting powei' under the influence of these rays. He finds the ’‘mlocity, with a 
})otential gradient of 1 volt per centim., to amount to about I'G centim. per second 
In air. 
In the present case, 
240 
Potential gradient = ^ ^ = 150 volts per centim. 
Velocity of carriers = 150 X 1‘G centim. per second. 
Time taken to travel across the air space = — ^ seconds 
^ loO X PG 
Thus even the carriers which have the greatest possible distance to travel reach one 
of the plates in less than ynofh of a second. This explains how no fogs were 
obtained when the expansion was made even a very short time after cutting off the 
Piontgen rays. 
Now, when the air was exposed to weak ultra-violet light in place of the Rontgen 
rays, the difference of potential being, as before, 240 volts, no diminiition in the 
number of the nuclei by the action of tiie electric field could be detected even 3 
seconds after the radiation was cut off. Even in three seconds the distance they have 
travelled under the influence of the electromotive force is therefore small compared 
with the thickness of the air layer. These nuclei therefore travel at least 300 times 
as slowly as those produced l)y liontgen rays undei* the same potential gradient. It 
is uidikely tliat this difference is due mainly to a difference in the size of the nuclei, 
tlie charge being the same ; for with ultra-violet light of the intensity used the two 
classes of nuclei are indistinguishable from one another, with resjDect to their power 
of enablina: condensation to take nlace unon them. There can he little hesitation in 
concluding that the nuclei produced throughout the volume of the moist air by the 
action of ultra-violet light differ from those produced by Rontgen rays in being 
uncharged. If any ions are present in air exposed to ultra-violet light they are 
exceedingly few in conq^arison with the uncharged nuclei which are at the same time 
produced. 
* Rutherford, ‘ Phil, l^lcag.,’ rol. 44, p. 422, 1897, 
T— second. 
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