412 MR. C. T. K. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
It is rainai'kable that the minimum supersaturation required to cause condensation 
on the ions should be the same in hydrogen as in air. It must, however, be 
remembered that in all these experiments watei' vapour is necessarily present ; and 
some of the ions may always be derived from it. It may be that those requiring the 
minimum exj^ansion to make condensation take place on them are produced from this 
source, when hydrogen is the gas under investigation. 
In hydrogen which is not exposed to Uranium rays or other nucleus-producing 
agent, no drops at all are produced even with Vojv^ as great as I'3 (see ‘ Phil. Trans.,’ 
loc. cit.). A small quantity of uranium oxide contained in a thin glass bulb, 16 
centims. away fiom the glass bulb forming the cloud chamber, was found to give quite 
a noticeable shower with Vo/Vi = 1 "277. An expansion apparatus filled with hydrogen 
thus forms a very sensitive detector of Uranium or X-rays. If we take the view 
already suggested, that these nuclei actually are identical with the ions, each 
individual ion being made visible on expansion by the formation of a visible drop 
around it, it is not surprising that the method should be more delicate than the 
photographic or electrical modes of detecting such rays. 
§ 5. Nuclei produced by Ultra-violet Light. 
In some experiments of Lenard and Wolfe (‘ Wied. Ann.,’ vol. 37, p. 443, 1889), 
light, rich in ultra-violet rays, was admitted through a quartz window into a vessel 
containing moist dust-free air. They found that if the air was allowed to expand 
after being exposed for some minutes to the light, a fog was produced showing that 
nuclei of some kind had been produced by the action of the ultra-violet rays. 
Similar results were obtained in steam-jet experiments. They regarded their 
experiments as proving that the ultra-violet rays caused the posterior surface of the 
quartz to disintegrate, the small })articles thrown off constituting the nuclei on which 
condensation took place. 
For the purpose of measuring the expansion required to make water condense on 
the nuclei produced in this way, I used apparatus identical with that made for the 
experiments with Kontgen rays, a quartz plate being, however, substituted for the 
aluminium closing the top of the cylindrical tube B. 
The quartz plate was attached in the same way as the aluminium plate, being- 
screwed tightly against an indiarubber band placed on the top of B. As in Lexard 
and Wolff’s expeiiments, the souice of the ultra-violet light was the spark between 
zinc terminals produced by an induction coil; a Leyden jar being inserted in the 
secondary circuit to brighten the spark. Short sparks of about 2 millims. in length 
were generally used. Cadmium terminals were substituted for zinc in many of the 
experiments, but with no great increase in the effect. The expansion apparatus was 
wrapped in tinfoil, provided with windows to enable the fogs to be seen, the quartz 
Itself being covered with wire gauze, placed on the brass diaphragm which held the 
quartz in position. 
