444 Mn. C. T. R. WILSON ON THE CONDENSATION NFCLEI PRODUCED IN 
they have time to grow appreciably. If, however, the electric field be removed 
suddenly, by short-circuiting the terminals, many of the carriers which have left the 
point of the wire may not have reached the walls before this process is completed, 
and their comparatively slow motion when the wire and walls are at the same 
potential enables them to persist for some time. The growth which then takes place 
is prohaldy the result of the condensation, upon tlie nuclei, of scune substance produced 
1)V the discharge. Tlie sul)stance may he nitric acid or H 2 O 0 . 
§ 10. Behaviour of the variotts kinds of Nuclei in an Electric Fjeld. 
It has already been suggested (‘ Camh. Phil. 80 c. Proc.,’ loc. cit.) that the nuclei 
requiring expansions between I'25 and 1'87 to make condensation take place on them 
are to be identihed with the ions, to wliich the conductivity of gases exposed to 
X-rays or Uranium-rays is due. The only evidence there furnished for this view was 
tlie fact that in ordinary moist air or other gases such nuclei were found to be present 
in exceedingly small numbers, while when the gas was made a conductor liy being 
exposed to X-rays or Uranium-rays, immense numbers of these nuclei could be 
detected. 
The experiments with the nuclei produced by the discharge from a pointed platinum 
wire, as well as with those which are produced by the exposure of a negatively 
charged zinc plate to ultra-violet light, support this view, at the same time pointing 
to the conclusion that in all these cases the carriers of the electricity are of the same 
kind. 
A difficulty, however, is introduced by the results obtained with air exposed to 
weak ultra-violet light or to the action of certain metals, for in l)oth cases nuclei are 
produced, rec[uiring, in ordei' tliat water may condense on tliem, a degree of super¬ 
saturation approximately the same as is required in the case of nuclei associated with 
conducting power in the gas. Now there is no evidence that either the presence of 
metals or exposure to Tiltra-violet ligfit causes air to act as a conductor of electricity. 
Pc might be thought tliat the great delicacy of the condensation method of detecting 
hue ions (eacli individual carrier lieing represented by a visible drop on expansion) 
was the cause of tliis apparent discrepancy, and that air under the conditions in 
(piestion really has conducting power, too small to lie detected by ordiiiary methods. 
Uie experiments to he descrilied, liowever, show that the nuclei produced Im the 
presence of metals, as well as those jiroduced by the action of ultra-violet light on 
moist air, differ from those present in air exposed to X-rays or Uranium-rays in not 
carrying a charge of electricity, or, to he more exact, in not being affected by an 
electric field. 
To compare the heliavioiu’ of the nuclei produced by the action of ultra-violet light 
on moist air witli that of the nuclei produced by Pontgen rays, the a])paratus shown 
in fig. 13 was used. The air is contained between two plates of a condenser, the 
