372 Electrodynamic Equations of a Moving Material Medium, $c. 



excitation of material dielectrics. This circumstance will perhaps 

 recall to mind Osborne Reynolds' theory of the dilatancy of granular 

 media, which explains that the discrete elements of such media 

 tend to settle down under the mutual influences of their neighbours so 

 as to occupy the smallest volume, and therefore any disturbing cause 

 has a tendency to increase the volume. 



In 80, on the influence of electric polarisation on ripple velocity, 

 the result stated for dielectrics should be doubled. It is to be 

 remarked that a horizontal dielectric liquid surface becomes unstable 

 in a uniform vertical electric field when the square of the total con- 

 tinuous vertical electric displacement exceeds the moderate value 



Kl . K . 2 i K2 t- w l) {Q*-/Qy T }* electrostatic units, T being the capil- 

 27r(K 2 KI)^ 



lary tension. For a conducting liquid instability ensues when the 



K 2 

 square of the surface electric density exceeds ~{pz pi)gT}* elec- 



2?r 



trostatic units. In exciting a dielectric liquid by the approach of an 

 electrified rod it must often have been noticed that when the rod is 

 brought too near, the liquid spurts out vigorously in extremely fine 

 filaments or jets : the fineness of the filaments may be explained, in 

 part at any rate, after Lord Rayleigh (' Phil. Mag.,' 1882, " Theory of 

 Sound," 364), without assuming an escape of electricity into the 

 liquid, as arising from the circumstance that it is only narrow crispa- 

 tions of the surface, and not extensive deformations, that become 

 unstable. 



The opportunity is taken to correct other errata in the memoir, 

 ' Phil. Trans.,' A, 1897, as follows : 



page 252, line 15, read Sir for 47r. 



253, 18, the factor c 2 is omitted. 

 , 297, 4, dele m. 



