Electromotive Forces in the Voltaic Cell. 173 
appear, therefore, that the reason why the results of metals by 
the ordinary ‘ contact-force’ experiments harmonize so well 
with the comparison by dipping beth metals in water or an 
oxidizing electrolyte, is not because the H.M.I. between a 
metal and a gas or an electrolyte is small, but because the pro- 
perties of air agree to a certain extent with those of ordinary 
electrolytes. Tor if the active component of the electrolyte is 
sulphur, the results are quite different ; and the same kind of 
difference occurs when hydrogen sulphide is substituted for air.”’ 
In their third paper*, therefore, we find Ayrton and Perry’s 
views changed, and they clearly state that their experiments, 
like all those based on inductive methods, leave the question 
of air-contacts quite undecided. 
They then go on to say :—‘ One way of determining the 
H.M.F. of contact in volts between a substance and air, and 
a way we hope shortly to employ, is to repeat all these exact 
contact-experiments in different gases [it is not quite true 
that this would give the results required, because it would 
only give differential effects ; very important to observe, no 
doubt, but not the same as observing the actual contact-force 
of air or of gas]. . . . We shall thus ascertain whether the 
contact-difference of potentials of a substance and a gas 
differs much for different gases. Qualitative experiments in 
this direction have already been made with very interesting 
results by Mr. Brown of Belfast, but his experiments differ 
from ours in not being quantitative.” [Only, as their ex- 
periments do not yet exist, Mr. Brown’s have still some 
value. 
In a xii. of Wiedemann’s Annalen t I find some in- 
teresting experiments by Schulze-Berge on contact-force 
between metals and gases. He uses a condenser and differ- 
ent gases, but the plates of his condenser are both of the same 
metal, and he coats one of the plates with a film of the gas, 
say chlorine, or hydrogen, or ozone, and leaves the other 
covered with air. To measure the potential-difference he 
employs the compensation method of Pellat, and his arrange- 
ment seems fairly satisfactory. But he does not explicitly 
enter on the question as to the seat of H.M.F. in the Volta 
experiment (except in a controversy with Professor Exner). 
He assumes that a contact-force between metals and metals 
and between metals and liquids has been established, and he 
wishes to extend it to the contact of a metal and a gas. 
* Phil. Trans. 1880. Oe 
+ Schulze-Berge, “Ueber die Elektricitatserregung beim Contact von 
Metallen und Gasen,” Ann. der Phys. u. Chem. xii. p. 298 (1881). 
