Electromotive Forces in the Voltaic Cell. 169 
which it is very difficult to settle by direct experiment. For 
if by using a chemically active gas instead of air, you get a 
positive result or change in the Volta effect, the answer from 
the other side is: “ Yes, of course, because your plates are 
corroded and coated with sulphide or chloride, or something 
whose contact-forces come in and modify everything.” Ii, 
on the other hand, you get a negative result when you substi- 
tute some inert gas like hydrogen for air, then it is objected 
that you have not removed the air-film which the plates had 
contracted from long standing in the air; and if you answer 
indignantly that you did, and that your hydrogen was per- 
fectly pure, it is replied with a sneer, ‘Oh yes, it is not so 
easy to get pure hydrogen as you seem to think.” 
Moreover, suppose a positive effect on changing the gas 
was established ; what then? Nothing is settled except that 
the metal /air contact-force is proved to be somewhat different 
from the metal / gas contact-force. There seems to be really 
no way of knocking contact-force on the head experimentally, 
and this probably because it is a reality: there really is a 
contact-force at every junction of dissimilar substances ; and 
the H.M.F. of a circuit, whether it be inductive or conductive, 
is always the sum of such contact-forces. I do not say that 
the contact-force at any given locality has the value ordinarily 
assigned to it as the result of experiment. 
The earliest attempt made to examine the question as to 
whether the Volta effect depended on the atmosphere was 
made by Pfaff* in 1829, who used dry and damp air, oxygen, 
nitrogen, hydrogen, carburetted hydrogen, and carbonic acid, 
_and he found that there was no difference so long as no visible 
chemical action occurred; butit must be noted that the oppo- 
sing faces of his plates were varnished. De la Rive, on the 
other hand, asserted that there was no Volta effect in the 
slightly rarefied air then known as “‘ vacuum.” 
In recent times Pellat has investigated the subject, and has 
come to a conclusion in agreement with Pfaff, viz. that the 
differences are very small. The metals used by Pellat were 
copper and zinc, and the gases were air dry and damp, dry 
oxygen, dry nitrogen, dry and pure hydrogen, dry and pure 
carbonic acid. He finds slight variations, but exceedingly 
slight, and such as Pfaff, Hxner,and Brown could hardly have 
detected. He says :—‘Au surplus il est fort probable que, si 
quelgues-uns des auteurs précédents avaient tenté les expé- 
riences que j’ai faites au sujet des gaz, ils auraient trouvé 
des résultats négatifs ; les faibles variations produites par le 
changement des propriétés du gaz que j’ai pu mettre nette- 
* Pfaff, Ann. de Chim. 2 ser. xli. p. 236. The metals he employed 
were copper, tin, and zinc. 
Phil. Mag. 8. 5. Vol. 19. No. 118. March 1885. N 
