206 
j\JE. J. lARMOR ON A DYNAMICAL THEORY OF 
Page 
Meclianical stresses and deformations in charged condenser’s (75, 76). 290 
Some practical applications and illnstrations of mechanical stress (77-84) : 
(i.) Refraction of a uniform field of electric force.292 
(ii.) Electric pressures in fiuid dielectrics.293 
(iii.) Electric expansion.294 
(iv.) Influence of electric jrolarization on the velocity of surface ripples .... 296 
(v.) Relation of electric excitation to vapour tension and fluid equilibrium . . . 297 
(vi.) Tractions on the interfaces of a divided magnetic cii-cuit . 298 
(vii.) Mutual influence of stress and magnetization . 298 
(viii.) Magnetic stress in a polarized solid sphere.299 
1. In two previous memoirs'^'' it has been explained, that the various hypotheses 
involved in the theory of electric and optical phenomena, wdiich has been developed 
by Faraday and Maxwell, can be systematized by assuming the tether to be a 
continuous, homogeneous, and incompressible medium, endowed with inertia and 
with elasticity purely rotational. In this medium unitary electric charges, or 
electrons, exist as point-singularities, or centres of intrinsic strain, which can move 
about under their mutual actions ; wdiile atoms of matter are in whole or in part 
aggiegations of electrons in stable orbital motion. In particular, this scheme provides 
a consistent foundation for the electrodynamic laws, and agrees with the actual 
relations between radiation and moving matter. 
An adequate theory of material phenomena is necessarily ultimately atomic. The 
older mathematical type of atomic theory which regards the atoms of matter as acting 
on each other from a distance by means of forces whose laws and relations are 
gradually evolved by observation and exjieriment, is in the present method expanded 
and elucidated by the introduction of a medium through whose intervention these 
actions between the material atoms take place. It is interesting to recall the circum¬ 
stance that Gauss in his electrodynamic speculations, wdiich remained unpublished 
during his lifetime, arrives substantially at this point of view ; after examining a law^ 
of attraction, of the Weberian type, between the “ electric particles,” he finally 
discards it and expresses his conviction, in a most remarkable letter to Weber,! that 
‘ Phil. Trans., 1894, A, pp. 719-822 ; 1895, A, pp. 695-743 ; referred to subsequently as Part I. 
and Part II. [In the abstract of the present Memoir, ‘ Roy. Soc. Proc.,’ 61, on p. 281, line 6, read 
2 -?i '2 q. foi- ; line 35 read f . for ; and on p. 284, line 18, read ?h/2c . E (1 - vi~) for 
E (1 - m2).] 
t Gauss, VVerke, Y., p. 629, letter to Webee of date 1845 ; quoted by Maxwell, “ Treatise ” II., § 861. 
After the present memoir had been practically completed, m}’ attention was again directed, through a 
reference by Zeeman, to H. A. Lokentz’s Memoir “ La Theorie Electromagnetique de Maxwell et son 
application aux corps mouvants,” Archives Neerlandaises 1892, in w’hich (pp. 70 seqq.') ideas similar 
to the above are developed. The electrodynamic scheme at w’hich he arrives is formulated diflerently 
from that given in § 13 infra, the chief difference being that in the expression for the electric force 
(P, Q, R) the term djdt (F, G, H) is eliminated by introducing the cethereal cP'iplacement 
1 his applies also to the later “Versuch einer Theorie ... in bewegten Korpern,’ 895. The author 
