420 Mli. C. T. R. WILSON ON THE CONDENSATION NUCLEI PRODUCED IN 
to very weak ultra-violet light, d(j not grow beyond the stage at which a four-fold 
supersaturatioii is required to make condensation take place upon them, grow under 
the influence of very strong radiation till they become large enough to scatter 
ordinary light. 
Tyndall, many years ago (‘ Phil. Trans.,’ vol. 160, p. 333, 1870), showed that when 
the mom i-efrangiljle rays from an arc lamp, such as were able to travei'se blue glass ijut 
not red glass, were concentrated within a tube containing air mixed with amyl-nitrite 
or certain other vapours, dense clouds were j^i’oduced. He was unable to obtain any 
such efiect with pure air and water only. The experiments to be described differ in 
no essential inspect from liis, except in the fact that the rays from the arc lamp wein 
allowed to travei’se no material such as glass, which is opaque to the ultra-violet rays, 
liefore enterin'^ the tube containing the moist air. Under these conditions, air con- 
taining water vapour only, shows the phenomena that were observed by Tyndall 
with other vapours. 
I have found the apparatus shown in fig. 5 convenient for exijeriments on this 
subject. It consists of a glass tul^e 34 centims. long, and 4 centims. in diameter, 
provided with a side tube near each end. The ends are closed by quailz lenses, 
which are fixed air-tight by means of indiarubber washers. They are pressed tightly 
against these indiarubber rings by means of two brass diaphiugms screwed together 
by means of three bolts just outside the tube. By means of the two Wolfi:‘ bottles, 
A, B, a current of filtered air can be driven through the a])paratus. The air is filtered 
before entering B, and again on leaving it. If, while A was fixed at some height 
above B, the stop-cock Tb remained closed while TT was open, the pressure in the 
apparatus was greater than the atmospheric pressure. On opening Tb the pressure 
was suddenly reduced to that of the atmosphere, and the expansion produced in this 
way was sufficient to cause condensation on ordinary dust particles. A small quantity 
of water w'as contained in the tube to keep the air saturated. To enable even faint 
clouds to Ije seen, the tube A\'as contained in a blackened box, o})en along one side, and 
