450 MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
required to make condensation take place on them may be less or more than is 
required for the ions according as their number is great or small, and those produced 
by ultra-violet light grow, if the rays are strong enough, till they become visible 
without expansion. To account for the growth of the nuclei under the influence of 
ultra-violet light, I have already suggested that some compound such as II 2 O 2 may be 
produced by the action of these rays on the nuclei. Vv^e may, perhaps, extend this 
idea somewhat, and regard also the nuclei j^roduced in air by weak ultra-violet light 
or by metals, as consisting of molecules of H 2 O 2 or of aggregates of molecules of H 2 O 
and H 2 O. 2 . ScHONBEiN, so long ago as 1866, found that hydrogen peroxide was 
produced by shaking togetlier amalgamated zinc, oxygen and water (‘ Journ. fiir 
Pr. Chem.,’ vol. 98, p. 65). There can he little doubt that the nuclei produced in the 
neighbourhood of metals have some relation to the active substance, hydrogen peroxide, 
“ active oxygen,” or whatever it may be, which is produced in many cases of .slow 
oxidation, and about which there has Ijeen so much coiitrover.sy. 
The question now arises in which cla.ss of nuclei, the charged or the uncharged, 
must we place those which always appear to be present in small numbers in moist air, 
giving rise to the rain-like condensation which takes place with expansions between 
the limits v^jv^ = 1"25 and Vojvi — U38. 
To decide this question the same apparatus as that used for the experiments last 
described was used, hut the zinc plate was replaced by one of sheet tin, because this 
metal appears to he inactive, that is, it produces no increase as far as can be detected 
in the number of nuclei present. The plate was, moreover, covered on both surfaces 
with wet Alter paper to prevent direct contact of the metal with the air. The 
thickness of the air layer was equal to U7 centims. ; an electromotive force of 
225 volts was used. 
The expansion used was that corresponding to a gauge reading of 187 millims. 
This gave a .slight shower, and no difference could be detected in its appearance 
whether the electromotive force was applied or not. This appears to indicate that 
these nuclei are not charged. It is, however, doubtful whether the tin, even when 
covered with wet filter paper, is absolutely inactive, and on that account, perhaps, not 
a great deal of weight can be attached to this experiment. If we assume that the 
ertect of the tin is negligible, there still remains the possibility that although the 
nuclei re(:[uiring expansions considerably exceeding the limit v-i/v^ — 1'25 are uncharged, 
the very few which require an expansion only very slightly exceeding this may be 
charged and identical with tliose produced by Hontgen inys. Otherwise we have the 
somewhat astonishing result that two quite different kinds of nuclei require absolutely 
tlie same degree of supersaturation, that, namely, corresponding to the expansion 
v.,jc] = I’25, in order that condensation should take place on them. To make experi¬ 
ments of the same kind with expansions only slightly exceeding the limit Vojv^ = 1‘25 
is difficult on account of the exceedingly small number of the drops. Apparatus on a 
much larger scale would be better for experiments on this point. 
