168 Prof. Oliver Lodge on the Seat of the 
In 1881 he observed a time-change (decrease) of the Volta 
effect at a copper-zine junction, and reckoned that at the first 
instant after cleaning the potential. difference would be as 
high as ‘9 Daniell, “‘ which,” he says, “agrees with J. Thom- 
sen’s determination of the difference of the heats of combus- 
tion of zinc and copper and oxygen.” He here gives a hint 
of holding a heterodox notion which I do not find in any 
other of his w ritings, and which I believe he abandoned, 
even if he ever really held it. 
In 1879 Brown tried a copper-nickel divided ring, substi- 
tuting HCl for air, and here also succeeded in obtaining a 
reversal of sign. He also arranged a divided ring of wet 
blotting-paper, and showed that there was a difference of 
potential when the two halves were touched with a zinc- 
copper couple (which is not remarkable); but he then goes 
on to draw a moral, and to say that the slit of the divided 
ring corresponds to the air-film, and the wet paper to the 
moisture-film in the ordinary Volta-condenser experiment. 
The film of moisture on the zine plate is thus shown to have 
a + charge, and that on the copper a negative. If it be 
objected that the better the plates fit, the better the manifes- 
tation of contact HE, it is to be replied that it is not to be sup- 
posed that there is no air between them anyhow (says Brown). 
Probably, he says, gas produces the difference of potential 
only so far as it forms a film on the surface. When a metal 
and a liquid are experimented on, it is probably really a two- 
fluid cell, the other fluid being that condensed on the surface 
of the metal. 
Brown thus goes strongly for the activity of the films, or 
condensed air-sheets, which certainly exist on the surface of 
solids, and which may play an important part in the matter; 
but he supposes that these films act by corroding or attacking 
the plates, and that such a film is necessarily existent between 
surfaces nominally in contact if any Volta effect is to be pro- 
duced ; so that if the metal faces really and truly touched all 
over, they would show no charge when separated. Moreover, 
he lays it down that the potential difference is only observed 
while chemical action is going on; but that so soon as it 
ceases, from any cause, at once the Volta effect ceases too. 
In all this I entirely differ from him; but his experiments are 
very interesting and much to the point. 
They cannot, however, be regarded as settling the question 
—the very important and fundamental question—as to whether 
the Volta effect depends on the atmosphere or medium sur- 
rounding the plates, or whether it is an absolute effect 
depending on contact alone. Hxperiments on this point are 
absolutely discordant; and it seems to be one of those points 
