258 
DE. HAEOLD A. WILSOX OX THE 
The leak is the same at 0'2 millim. as at OT millim., and it was verified carefully 
that no change occurred even on pumping down to O'OOl millim. The numbers in 
brackets refer to the order in which the measurements were made. The following 
talde gives the values of the leaks in amperes and tire temperatures deduced from the 
above numbers :— 
Temperature 
on platinum scale. 
Temperature 
on Centigrade scale. 
Leak in amperes. 
Amperes per square centi¬ 
metre of platinum. 
(1) 
0 
1319 
o 
137.5 
5-8 X 10-3 
1-57 X10-s 
(2) 
1344-.5 
1408-5 
12-7 X10-3 
3-43xl0-« - 
(3) 
1370 
1442 
27-6 X10-3 
7-46 X10-s 
(t) 
1395‘5 
1476 
56-4X10-3 
15-2 X10-® 
(-5) 
1421 
1510-5 
119-6 X10-3 
32-3 xlO-s 
(6) 
1446-.5 
1545 
236 X10-3 
63-8 xlO-8 
(7) 
1472 
1580 
473 X10-3 
128-0 xlO-8 
It will 1)6 observed that as the temperature increases by nearly equal increments 
tlie current increases l^y nearly equal factors. 
A theoretical proof of the formula x = will now he given, and then it will 
l)e shown that the aljove results can be represented ])y this formula. The emission 
of negative corpuscles l)y hot platinum is analogous to the evaporation of a liquid, 
and whether the corpuscles come out of the metal or are produced at its surface, the 
number produced per second per unit area of platinum surface may be regarded as 
analogous to the number of molecules emitted per second l)y unit area of a liquid. 
If [) is tiie vapour pressure, of a liquid at the absolute temperature 6, and L its 
latent heat of evaporation per gramme molecular weight, then L = (iq — v-^6 
where = volume of vapour and — volume of liquid. Neglecting iq and putting 
V., = V\j6/p we get L = . ilpjdd. Let the internal work done in evaporating the 
liquid Ije Q and suppose Q = Qg fi- a9, where a is some constant, then L = Q + so 
that we have 
LIq d" "k = Ivd~/yn dpjdO 
or 
Qo + (Id’ + 
dd = 
Jldp 
r 
Hence if is the vapour pressure at a tenqjerature 6^, and that at (9o, 
n p.d 
Ot)' * — ‘ 
= Qo 
^1 
Now = 6mVN, wliere m is the mass of a molecule, Y the square root of the mean 
square of the velocities of all the molecules, and N the number leaving each square 
centimetre of the liquid surface per second and h a constant. But Y^ is proportional 
