G. BF. Firzcrratp—On the Superficial Tension of Fluids. 99 
volume would remain unaltered, and besides, their very complicated structure would 
then seem unnecessary. 
Nore appED NovemBer 26, 1878.—It may seem doubtful whether my assumption, that an electromo- 
tive force of contact exists at the surface of separation.of a liquid and a gas is justified, especially as 
Messrs. Ayrton and Perry seem to assert the contrary. Nevertheless in a paper published in the 
Philosophical Magazine for August, 1878, Mr. Brown details some experiments which prove conclusively 
that there exists an electromotive force of contact at the surface of separation of a metal and a gas, and 
probably the same is true of a liquid and a gas. In the case he mentions the direction of the electromo- 
tive force between copper and iron, was reversed by substituting sulphide of hydrogen for air, and this 
shows that these electromotive forces are at least comparable to those which have been hitherto assumed 
to be due to the contact of the metals with one another. 
Tt seems probable that the cohesion of molecules in matter and of groups of atoms in binary compounds, 
are phenomena of the same kind. 
Considering the enormous liquid surface produced in the form of spray by Armstrong’s electrical 
machine, it seems reasonable to explain its efliciency by the electrical displacements accompanying the 
enlargement and diminution of liquid surfaces. 
