182 Prof. Oliver Lodge on the Seat of the 
Professor Exner, to strengthen his position, adduces a large 
number of very simple experiments (such as connecting first 
one Daniell and then two Daniells to an electrometer, and 
observing that in the second case the deflection is double the 
first), and from them i obtains equations proving algebrai- 
cally that Zn/Cu=0. Considered as conundrums these 
equations are ingenious, but it is a waste of time seriously to 
discuss them as Herr Julius has done in an elaborate manner. 
To suppose that such everyday experiments as these are in 
direct contradiction of the contact theory is scarcely compli- 
mentary to the great men who have held, and who still hold, 
that view. 
Dr. C. G. Knott, in 1879*, examined the contact-force 
between plates of the same metal at different temperatures, 
using the condenser or Kohlrausch method. He found that 
iron, copper, zine, and probably tin, were negative when hot 
to the same metals cold ; and the effect. increases uniformly 
with temperature. But it is permanent, remaining after the 
hot plates have cooled down; hence it must be due to 
oxidation. 
A slow oxidation proceeds with time alone. Time-curves 
are logarithmic like cooling-curves, and the most oxidizable 
metal varies most quickly both for time-variation and tempe- 
rature-variation. ‘There seems to be a surface-condition of a 
metal proper to each temperature which no polishing can 
change, for it establishes itself in a few seconds after cleaning, 
and only changes with temperature. 
Mr. 8. Lavington Hart, in 18517, describes a mercury- 
dropper where the mercury is contained in a funnel, and is 
connected with an electrometer by an iron rod dipping into 
it. The drops form inside an iron inductor, and they fall 
negatively charged. Mr. Hart so far ignores any Volta force 
that he considers the arrangement as an inversion of Lipp- 
mann’s electrometer, the advancing drops being oxidized. It 
can plainly be regarded, however, as a mere Fe / Hg contact 
arrangement; and that is what I suppose it to be. He makes 
two interesting modifications : the first is to replace the air 
round the dropping mercury by coal-gas ; the electrical effect 
is then zero. ‘This is interesting, because the exuding drops 
of mercury, unlike most pieces of metal, expose to the coal-gas 
a virgin surface which has probably contracted no condensed 
air-sheet: only coal-gas is a rather sophisticated substance for 
* Knott, Proc. R. 8. Edinb. 1879-80, No. 105, p. 362. 
+ Hart, Brit. Assoc. York, p.555; and Phil. Mag. November 1881, 
ser. 5, xil. p. 524, 
