424 
mi. C. T. II. WILSON ON THE CONDENSATION NUCLEI PEODUCED IN 
was broug'lit to a focus near the middle of the tuhe, a fog developed in less than two 
minutes. 
Expansion experiments were made with oxygen with the same apparatus as was 
used in the experiments on air. The results obtained were identical with those 
obtained in the former experiments. The oxygen was obtained by heating potassium 
permanganate. With weak radiation nuclei were produced requiring, however long 
an exposure might be made, a pressure fall of 15 centims. or more; while, when the 
radiation was stronger, fogs were produced with comparatively slight exj^ansions, the 
expansion required depending on the time of exposure. Finally, with sufficiently 
strong ultra-violet rays, visible fogs were obtained without expansion. 
For the purpose of making experiments with oxygen as pure as could he obtained ; 
and witliout any danger of contamination l^y vapours, which might be present if any 
indiarubber or cements were used in attaching the quartz ])late, the apparatus shown 
in fio’. 7 was used. 
Fig. 7. 
The vei'tical tube A, in which the gas was exposed to the action of the rays, was 
IG centims. long and 3 centims. in diameter. Its open end was carefully ground flat 
and closed hy a quartz plate, which was simply placed upon it, mercury being then 
j)oured into the wooden collar surrounding it to make a tight joint. The tube was 
exhausted before filling the collar with mercury. 
A small quantity (less than 1 cub. centim.) of well boiled distilled water was placed 
in the tiibe immediately before closing it with the quartz plate. The water was 
drawn up into a pipette while still boiling and run into the apparatus after cooling 
slightly. The apparatus was then pumped out hy connecting to a water pump. 
The oxygen was prepared by beating potassium permanganate, which had been 
twice recrystallised, and then heated in an open dish till the greater part of it fell to 
j)owder. Between the tuhe which contained the permanganate and the rest of the 
