Electromotive Forces in the Voltaic Cell. 171 
In all the above gases he has also studied the effect of varying 
the pressure. Lowering the pressure slightly increases the 
observed difference of potential; but the change lags a little 
behind the pressure-variation. Damp and dry air behave in 
the same way. In oxygen the effects of pressure are rather 
betier marked. Nitrogen gives nearly the same numbers as 
air; but after it has been in for some time, the numbers are 
slightly lower than at first. Hydrogen gives a little greater 
effect than even rarefied air; rarefying hydrogen does not 
alter it much. Carbonicacid gives the same numbers as rare 
air or dense hydrogen. As for liquids, plates wetted with 
alcohol give the same result as if immersed in plain air. 
Von Zahn* also tried a condenser in various gases, and 
found no difference; but when he tried a platinum-zinc con- 
denser in the highly rarefied air now known as vacuum with 
some melted sodium in a branch tube to absorb all the oxygen, 
the Volta effect was diminished, and only represented a po- 
tential difference of half a Daniell. Iam not clear whether 
sodium can be trusted to ultimately absorb every trace of 
oxygen; but I should judge it would take a very long time: 
and as to rarefaction, dividing the numbers of molecules in a 
vessel by a million or two leaves them quite numerous enough 
to accomplish anything they want. 
Sir W. Thomson has also made experiments in different 
gases with negative results}. These experiments are not de- 
scribed in detail, but they were made with the apparatus shown 
in fig. 10. 
The views of Ayrton and Perry on the subject of the effects 
of atmosphere underwent modification between their first 
paper and their third. In their first paper they say they have 
good reason to believe that there is no great difference of 
potential between a metallic or liquid surface and the air in 
contact with it 
Clerk Maxwell, in a letter to the ‘ Electrician,’ { pokes fun 
at them for this, saying:—“‘A statement like this, coming 
from men whose scientific energy is threatening to displace 
the centre of electrical development, and to carry it quite 
out of Hurope and America to a point much nearer Japan, 
is worthy of all attention, even without an explicit statement 
of their ‘ good reason.” But Mr. J. Brown has shown (Phil. 
Mag. August 1878) by the divided-ring method of Sir W. 
Thomson, that whereas copper is negative with respect to iron 
in air, it is positive with respect to iron in H,S. It would 
* Memoir quoted below. 
+ Thomson, Brit. Assoc., Trans. of Sect., Swansea (1880), p. 494. 
_ ‘Electrician,’ April 26, a : 
