404 MR. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
and I‘38 was taken as indicating the j^resence of some other kind of nuclei than the 
molecules of gas or vapour. 
It was further found that when the gas was exposed to even weak Piontgen 
radiation, comparatively dense fogs were obtained when m/fi exceeded I‘25 (the 
supersaturation lieing then approximately four-fold), no condensation taking jDlace 
with smaller expansions. Thus, exposure of the gas to Ptontgen rays causes nuclei to 
be produced, requiring a definite degree of supersaturation in order that water may 
condense upon them. Later (‘Proc. Camb. Phil. Soc.,’vol. 9, p. 333, 1897) it was 
found that nuclei, requiring exactly the same minimum expansion to catch them, are 
produced in air by the action of uranium rays. 
In the paper just referred to the conclusion was drawn that the nuclei produced 
by X-rays and uranium rays are identical with one another, as well as with those 
always present in small numbers in moist air, and causing the rain-like condensation 
which results when m/'^i lies between 1'25 and 1'38. It was also there suggested 
that these nuclei are to be identified with the “ ions,” to the j^resence of which the 
conducting power of gases exposed to X-rays or uranium rays is due. 
The primary object of the experiments described in the present paper was the 
study, by comparison of their efficiency as nuclei of condensation, of the carriers of 
the electricity in gases, when these are made by any of the known methods to be 
capable of allowing the passage of electricity through them. In the course of the 
work certain other kinds of nuclei were unexpectedly met with, which appear not to 
be associated with any conducting power in the gas. The method by which nuclei 
carrying a charge of electricity were distinguished from such uncharged nuclei is 
described in § 10. 
I must explain here the meaning to be attached to certain expressions frequently 
used throughout this paper to avoid circumlocution. I have spoken of the expansion 
required to “ catch ” nuclei, meaning the expansion required to cause water to con¬ 
dense on such nuclei. The expressions “larger” a,nd “smaller” are often used of 
nuclei instead of “ requiring a less degree of supersaturation,” or “ requiring a greater 
degree of supersaturation, in order that condensation may take place on them.” 
Nuclei are often said to “grow” when they become larger in the sense just defined. 
It is probable that the expressions “larger” and “smaller” may be taken literally, 
without error ; for we may suppose such nuclei to be very small drops of water, 
whicli are able to persist in spite of their small size, because the effect of the 
curvature of the surface in raising the equilibrium vapour-pressure is balanced by the 
opposite effect produced by tlie drop either iDeing charged with electricity or con¬ 
taining some suljstance in solution. An increase in the charge of electricity or of 
the (piantity of dissolved substance, either of which woidd increase the efficiency of 
the drop as a condensation nucleus, would also result in an immediate increase in the 
size of the nucleus necessary for equilii)rium. 
