437 
GASES BY THE ACTION OF EONTGEN EAYS AND OTHEE AGENTS. 
Zinc Plate freshly polished. Depth of water over the quartz 1'5 centim. Thickness 
of hydrogen layer = 2'3 centims. ; barometer =749, temperature =15° C. 
:= 13 millims. ; ??i = 1 niillim. 
Gauge reading (in 
millims.) = j)- 
E.M.F. 
Eesult of Expansion. 
Zinc positive. 
Zinc negative. 
Short circuit. 
171 
6 Leclanche cells 
1 or 2 drops 
Fog 
152 
6 
0 
Shower 
161 
6 „ ,, 
1 or 2 drops 
Fog 
1 or 2 drops 
161 
1 
1 or 2 drops 
Slight fog 
161 
O 
^ 
Dense fog 
161 
6 „ 
Fog no denser 
i 
1 
161 
20 secondaries 
Much less dense 
' 
161 
120 
1 or 2 di'ops 
1 or 2 th’ops 
1 01 ’ 2 drops 
151 
6 Leclanche cells 
0 
Very few drops 
145 
6 
0 
1 or 2 drops 
143 
6 „ ,, 
0 
0 
Gauge reading, when expansioii is just sufficient to make condensation take place 
on the nuclei due to the negative electrification = 145 millims. 
o 
Corresponding value of = 1 '247. 
B 
TT — p 
It will be noticed that the expansion required to make condensation take place on 
the nuclei, produced by the action of ultra-violet light on a negatively electrified zinc 
plate, in air or in hydrogen, is identical with that required in the case of the nuclei 
produced by X-rays and Uranium rays. 
It is only when the zinc plate is negatively electrified that there is any action of 
this kind. 
With the zinc at the given height above the surface of the water the density of the 
fog produced in hydrogen by a given expansion is a maximum with a comparatively 
small difference of potential between the zinc and water. The maximum number of 
drops is obtained with a difference of potential produced by 6 Leclanche cells or less. 
With the much stronger field, produced by 120 secondary cells (= 240 volts) only a 
few scattered drops were seen, no more numerous than were obtained with the same 
expansion when the zinc plate was positively charged, or wlien the terminals leading 
to the zinc and water respectively were connected together by a wire. 
The diminution of the number of the drops as the electromotive force is increased is 
easily understood, for when the electric intensity is more than sufficient to remove all 
the carriers from the zinc as fast as they are produced by the ultra-violet light, then 
the total number of the carriers which cross from one plate to the other in a given 
time must remain constant, being equal to the number produced in that time by the 
ultra-violet light. The velocity of each carrier, however, is proportional to the electric 
