THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
7(J9 
the medium. This rotation is the representative of electric force, or rather its effect 
electric displacement, iii the medium ; and, in so far as it is not along the interface, 
its line integral from one body to the other will account for a difference of electric 
potential between them. The electric force must be very intense, as in fact are ail 
molecular forces, in order to give rise to a ffnite difference of potential in so short a 
range. If the bodies in contact are conductors, instead of dielectrics, similar con¬ 
siderations apply, but now the internal equilibrium of each conductor requires that 
the potential shall be uniform thr(jughout it; therefore the surface stress must so 
adjust itself that the difference of potentials between the conductors is the same at 
each point of the interface. 
The contact phenomena between a solid and a liquid are different from those 
between two solids ; for the mobility of the liquid allows, after a sufficient lapse of 
time, an adjustment of charged dissociated ions along its surface so as to ease off* the 
internal stress ; and thus the boundary of the liquid becomes completely and somewhat 
permanently polarized. If we consider for example blocks of two metals, copper 
and zinc, separated by a layer of water, the electric stress in the interior of the 
water becomes null, and the difference of potential between tlie two metals is the 
difference of the potentiahdifterences between them and water. That will not be 
the same as their difference of potential when in direct contact; but according to 
Lord Kelvin’s experiment it is sensibly the same as the difference between them and 
air,—owing in Maxwell’s opinion to similarity in the chemical actions of air and 
water. In this experiment the electric stress is not transmitted through either of the 
metals; its seat is the surrounding aether, and the function of the metals is so to 
direct it, owing to the absence of aethereal elasticity inside them, that the axis of 
the rotation of the aether shall be, at all points of their surfaces, along the normal. 
07. Let us imagine a Volta’s chain of different metals, forming a complete circuit, 
to be in electric equilibrium, as it must be, in the absence of chemical action and 
differences of tenqierature, by the principles of Thermodynamics. There is no electric 
stress transmitted through any metallic link of the chain ; the stress is transmitted 
through the portion of the aether surrounding each metal, consisting in part of the 
interfacial layers separating it from the neighbouring metal, and in part of the 
atmosphere which surrounds its sides. In the equilibrium condition the potential 
in the aether all round the surface of the same metal is uniform ; and this uniformity 
applies to each link in the chain. Therefore the sum of the very rapid changes of 
potential which occur in crossing the different interfaces, is, when taken all round 
the chain, strictly null: and we are thus led to Volta’s law of potential-differences 
for metallic conductors. Now suppose some cause disturbs this equilibrium, say the 
introduction of a layer of an electrolyte at an interface; this will introduce a store of 
chemical potential energy which can be used up electrically, and so equilibrium need 
no longer subsist at all. The uniformity of potential in the dielectric all round the 
surface of each metal will be disturlied, and a change of the electric displacement, 
MDCCCXCIV.—A. 5 E 
