436 MR. C. T. R. ^YILSOX OX THE COXDEXSATIOX XE'CLEI PRODEXED iX 
Gauge reading when the effect of the negative charge is first detected = 151 
inillims. 
Corresponding value of ^—■ 1'252. 
In the second series, partly owing to the greater thickness of the air layer, and the 
consequent greater chance of drops being seen, partly also on account of greater 
activity of the zinc surface, or greater intensity of the ultra-violet rays, drops were 
seen, with the expansions used, even when the zinc was positively charged or neutral. 
The change produced when the charge is negative is however well marked. It will 
be seen that, although drops are formed even when the expansion is below the lunit 
v.Jvi = 1’25, the effect of the negative electrification first becomes manifest at that 
point. It will be remembered that the nuclei produced by metals or by ultra-violet 
light do not show any very definite limit in the least expansion required to catch 
them. 
Experiments were also made in which all conditions were the same as in the 
experiments just described with this exception, that the zinc plate was covered on 
the side exposed to the ultra-violet light with wet filter paper. Under these 
conditions, as is well known,* it ceases to be capable of allowing negative electricity to 
escape under the influence of ultra-violet light. 
The result of a given expansion was now found not to depend on the sign of the 
charge on the zinc plate, the ajopearance of the showers or fogs being the same 
whether the zinc was jjositive, negative, oi' uncharged. 
Hydrogen. 
The phenomena are more easily studied in hydrogen than in air, the eflect of ultra¬ 
violet light throughout the volume of the gas being so very slight that quite strong 
radiation may be used. 
The hydrogen was obtained by heating palladium vdiich had been charged with the 
gas. The gas was first allowed to pass through the apparatus at a pressure of a few 
centimetres of mercury. The tube E was prolonged downwards and passed through 
the cork of a small wash-bottle containing distilled water, through which the gas had 
to bubble on its way to the pump. A convenient quantity of this water was finally 
admitted into the apparatus by closing the tube leading to the pump, and allowing a 
little air to enter the wash-bottle. The clip betv'een the wash-bottle and the 
expansion apparatus was then closed and the hydrogen brought to atmospheric 
pressure by heating the palladium. A })arallel beam of ultra-violet light Avas used, 
the zinc-spark being placed at the focus of a quartz lens (focal length = 6 centims.) 
The beam of light Avas just wide enough to illuminate the AA'hole of the zinc plate. 
* Stoletoav, ‘Coinptes Reudiis,’ a’oI. lOG, p. 1593, 1888. 
