434 ^IR. C. T. R. WILSOX OX THE COXDEXSATIOX NUCLEI PRODUCED IN 
investigate by its means this effect in air. For, as has already been sho^vn, not only 
does ultra-violet light produce nuclei in moist air in the absence of any zinc plate, 
negatively charged or otherwise, but a zinc plate introduces nuclei into the 
surrounding air in the absence of both electrification and ultra-violet light. These 
two effects are apt to disguise the one looked for. 
By using very weak ultra-violet light, however, it was found possible to 
demonstrate the production of condensation nuclei b}^ the action of the light on a 
negatively charged zinc plate. 
Fig. 11. 
The ultra-violet light (from a zinc spark) entered the apparatus (shown in fig. 11) 
through a quartz plate covered with 2 or 3 centims. of water, the surface of which 
served as one plate of a condenser, a zinc plate placed about 1 centim. above the 
water serving as the other. The condensation from the action of the ultra-violet 
light alone, or of the zinc alone, was too slight to be detected with a pressure fall 
below about 18 centims. ; the short distance which the few drops that are doubtless 
produced had to fall, causing them to be overlooked. 
On allowing the ultra-violet light to strike the zinc plate, and using a pressure fall 
of between 15 and 18 centims., a fog was obtained if the zinc plate was made 
negative (a difference of potential of a few volts being applied between the plates of 
the condenser) ; while no condensation could be detected if the zinc was positive or 
uncharged. The experimental details are given below. 
The general construction of the cloud-vessel will be understood by reference to the 
figure (fig. 11). The zinc plate was soldered to a brass rod, which passed through 
