PEOFESSOE CLEEK MAXWELL ON THE ELECTEOMAGNETIC FIELD. 
463 
as the displacement is increasing or decreasing. In the interior of the dielectric there 
is no indication of electrification, because the electrification of the surface of any molecule 
is neutralized by the opposite electrification of the surface of the molecules in contact 
with it ; but at the bounding surface of the dielectric, where the electrification is not 
neutralized, we find the phenomena which indicate positive or negative electrification. 
The relation between the electromotive force and the amount of electric displacement 
it produces depends on the nature of the dielectric, the same electromotive force pro- 
ducing generally a greater electric displacement in solid dielectrics, such as glass or 
sulphur, than in air. 
(12) Here, then, we perceive another effect of electromotive force, namely, electric 
displacement, which according to our theory is a kind of elastic yielding to the action 
of the force, similar to that which takes place in structures and machines owing to the 
want of perfect rigidity of the connexions. 
(13) The practical investigation of the inductive capacity of dielectrics is rendered 
difficult on account of two disturbing phenomena. The first is the conductivity of the 
dielectric, which, though in many cases exceedingly small, is not altogether insensible. 
The second is the phenomenon called electric absorption *, in virtue of which, when the 
dielectric is exposed to electromotive force, the electric displacement gradually increases, 
and when the electromotive force is removed, the dielectric does not instantly return to 
its primitive state, but only discharges a portion of its electrification, and when left to 
itself gradually acquires electrification on its surface, as the interior gradually becomes 
depolarized. Almost all solid dielectrics exhibit this phenomenon, which gives rise to 
the residual charge in the Leyden jar, and to several phenomena of electric cables 
described by Mr. F. Jenkin f. 
(14) We have here two other kinds of yielding besides the yielding of the perfect 
dielectric, which we have compared to a perfectly elastic body. The yielding due to 
conductivity may be compared to that of a viscous fluid (that is to say, a fluid having 
great internal friction), or a soft solid on which the smallest force produces a permanent 
alteration of figure increasing with the time during which the force acts. The yielding 
due to electric absorption may be compared to that of a cellular elastic body containing 
a thick fluid in its cavities. Such a body, when subjected to pressure, is compressed by 
degrees on account of the gradual yielding of the thick fluid ; and when the pressure is 
removed it does not at once recover its figure, because the elasticity of the substance of 
the body has gradually to overcome the tenacity of the fluid before it can regain com- 
plete equilibrium. 
Several solid bodies in which no such structure as we have supposed can be found, 
seem to possess a mechanical property of this kind $ ; and it seems probable that the 
* Faraday, Exp. Ees. 1233-1250. 
t Eeports of British Association, 1859, p. 248 ; and Eeport of Committee of Board of Trade on Submarine 
Cables, pp. 136 & 464. 
t As, for instance, the composition of glue, treacle, &c., of which small plastic figures are made, which after 
being distorted gradually recover their shape. 
