492 
PROFESSOR CLERK MAXWELL ON THE ELECTROMAGNETIC FIELD. 
The quantity v may be determined by experiment in several ways. According to the 
experiments of MM. Weber and Kohlrausch, 
0=310,740,000 metres per second. 
(81) It appears from this investigation, that if we assume that the medium which 
constitutes the electromagnetic field is, when dielectric, capable of receiving in every 
part of it an electric polarization, in which the opposite sides of every element into 
which we may conceive the medium divided are oppositely electrified, and if we also 
assume that this polarization or electric displacement is proportional to the electro- 
motive force which produces or maintains it, then we can show that electrified bodies 
in a dielectric medium will act on one another with forces obeying the same laws as are 
established by experiment. 
The energy, by the expenditure of which electrical attractions and repulsions are pro- 
duced, we suppose to be stored up in the dielectric medium which surrounds the electri- 
fied bodies, and not on the surface of those bodies themselves, which on our theory 
are merely the bounding surfaces of the air or other dielectric in which the true springs 
of action are to be sought. 
Note on the Attraction of Gravitation. 
(82) After tracing to the action of the surrounding medium both the magnetic and 
.the electric attractions and repulsions, and finding them to depend on the inverse square 
of the distance, we are naturally led to inquire whether the attraction of gravitation, 
which follows the same law of the distance, is not also traceable to the action of a 
surrounding medium. 
Gravitation differs from magnetism and electricity in this ; that the bodies concerned 
are all of the same kind, instead of being of opposite signs, like magnetic poles and 
electrified bodies, and that the force between these bodies is an attraction and not a 
repulsion, as is the case between like electric and magnetic bodies. 
The lines of gravitating force near two dense bodies are exactly of the same form as 
the lines of magnetic force near two poles of the same name ; but whereas the poles are 
repelled, the bodies are attracted. Let E be the intrinsic energy of the field surrounding 
two gravitating bodies M,, M 2 , and let E' be the intrinsic energy of the field surrounding 
two magnetic poles m 1? m 2 , equal in numerical value to M,, M 2 , and let X be the gravi- 
tating force acting during the displacement lx, and X' the magnetic force, 
X^=SE, X'lx=l E'; 
now X and X' are equal in numerical value, but of opposite signs ; so that 
IE=-IE', 
or 
E=C — E' 
=C-2i(«>+/3‘+/)<iV J 
