450 Mr. J. Larmor. A Dynamical Theory of [Dec. 7, 



exist on the atom, supplies the attracting agency. Furthermore, the 

 law of attraction between these charges is that of the inverse square 

 of the distance, and between the atomic currents is that of the inverse 

 cube ; so that, as in the equilibrium state of the molecule these forces 

 are of tbe same order of intensity and counteract each other, the first 

 force must have much the longer range, and the energy of chemical 

 combination must therefore be very largely electrostatic, due to the 

 attraction of the ions, as von Helmholtz has clearly made out from 

 the phenomena of electrolysis and electrolytic polarisation. 



But in this discussion of the phenomena of chemical combination 

 of atoms we have been anticipating somewhat. All our conclusions, 

 hitherto, relate to the aether, and are therefore about electromotive 

 forces. We have not yet made out why two sets of molecular aggre- 

 gates, such as constitute material bodies, should attract or repel each 

 other when they are charged, or when electric currents circulate in 

 them; we have, in other words, now to explain the electrostatic and 

 electrodynamic forces which act between material systems. 



Consider two charged conductors in the field ; for simplicity, let 

 their conducting quality be perfect as regards the very slow displace- 

 ments of them which are contemplated in this argument. The 

 charges will then always reside on their surfaces, and the state of the 

 electric field will, at each instant, be one of equilibrium. The magni- 

 tude of the charge on either conductor cannot alter by any action 

 short of a rupture in the elastic quality in the aether; but the 

 result of movement of the conductors is to cause a re-arrange- 

 ment of the charge on each conductor, and of the electric displace- 

 ment (/, #, Ti) in the field. Now the electric energy W of the system 

 is altered by the movement of the conductors, and no viscous forces 

 are in action ; therefore the energy that is lost to the electric field 

 must have been somehow spent in doing mechanical work on the 

 conductors ; the loss of potential energy of the electric field reappears 

 as a gain of potential energy of the conductors. We have to consider 

 how this transformation is brought about. The movement of the con- 

 ductors involves, while it lasts, a very intense ideal flow of electric dis- 

 placement along their surfaces, and also a real change of displacement 

 of ordinary intensity throughout the dielectric. The intense surface 

 flow is in close proximity with the electric flows round the vortex 

 atoms which lie at the surface ; their interaction produces a very in- 

 tense elastic disturbance in the medium, close at the surface of the 

 conductor, which is distributed by radiation through the dielectric 

 as fast as it is produced ; the elastic condition of the dielectric, on 

 account of its extreme rapidity of propagation of disturbances com- 

 pared with its finite extent, being always extremely nearly one of 

 equilibrium. It is, I believe, the reaction on the conductor of these 

 wavelets which are continually shooting out from its surface, carry- 



