56 Mr. J. Larmor. Theory of Electrodynamics, [June 2, 



It was shown in the paper above referred to that this hypothesis, 

 adopted by Helmholtz, led by itself without any necessity for further 

 assumptions which its author introduced on various grounds to the 

 undulatory propagation of electromotive disturbances across dielectric 

 media, with the same transverse type of waves as constitute light. 

 There will nftually be, in addition, a disturbance of a g'wcm'-compres- 

 sional character, which, on the more special hypothesis of Helmholtz, 

 is also propagated as a wave of permanent type, but with different 

 velocity. The electric undulations of transverse type have been 

 detected by Hertz ; and the balance of evidence, from the experiments 

 of different authors, seems to point to the conclusion that their 

 velocities in different media are inversely as the square roots of 

 the specific inductive capacities. Should this be fully verified, it 

 would follow demonstratively that the Helmholtz hypothesis must be 

 restricted to the special form which represents the Maxwell displace- 

 ment theory; and the general equations of electrodynamics, or rather 

 the electromotive part of them, will be definitely established. 



2. The object here proposed is to pass on from the electromotive 

 to the ponderomotive properties of the electric field, and examine 

 whether the latter lend any strength to the conclusions derived from 

 the former. Instead of a kinetic phenomenon like undulatory propa- 

 gation, we shall now consider the static phenomena of the stress 

 produced in the material of a dielectric by its excitation ; and, to 

 avoid the complexity, both optical and mechanical, introduced by the 

 elasticity of solids, we shall consider solely liquid dielectrics, on 

 which a very valuable series of experiments has been made by 

 Quincke.* The mechanical stress in a fluid depends on one variable, 

 the intensity of the hydrostatic pressure, and therefore may be con- 

 nected immediately with the distribution of the energy in the medium, 

 by means of the principle of work. 



The arguments for the actual existence of a stress of the Maxwell 

 type may be exhibited in a synthetical manner as follows : Consider 

 a condenser formed by two closed conducting sheets, one inside the 

 other; and imagine the equipotential surfaces to be traced in the 

 excited fluid dielectric between them. It is a matter of experimental 

 knowledge that there is a traction on each face, acting inwards, and 

 equal, at any rate approximately, to KF 2 /87r per unit surface, where 

 F is the electric force. Now the electric potential and therefore the 

 state of the dielectric fluid, will be in no wise altered if we imagine a 

 very thin stratum along one of the equipotential closed surfaces to 

 become conducting. There will therefore be a normal traction given 

 by the same formula, on each element of area of this surface. If this 

 traction is an affair transmitted across the medium, the transmitting 

 stress must be a tension KF 2 /87r along the lines of force. To form an 

 * ' Wiederaann's Annalen,' vol. 19, 1883. 



