454 Mr. J. Larmor. A Dynamical Iheory of [Dec. 7, 



sufficiently close together, and by taking the dimensions of the con- 

 ducting atom sufficiently small. 



As regards the second of these hypotheses, it is to be observed that 

 the moment of electric induction in a conducting atom depends only 

 on its size, and not on the intensity of its free electrification ; for the 

 case of conducting spherules the electric moment produced by the 

 action of an electric force F is 3F/4?r multiplied by the total volume 

 of the atoms, and this would give a dielectric inductive coefficient 

 equal to three times the ratio of the aggregate volume of the atoms 

 to the whole volume of the region, a result which is, in any case, far 

 too small to represent the facts, and may easily be so small as to be 

 quite negligible, so as to leave the current practically circuital. 



But if we add on to this the first assumption, no room will be 

 left for the explanation of the pyro-electricity and piezo-electricity 

 of crystalline media, by changes of orientation of polar molecules 

 due to changes of temperature or to applied pressure. If this very 

 rational explanation is to be retained, we are driven to assume that 

 the electric force of the field does not sensibly alter the orientation 

 of a molecule, which would then be wholly controlled by the internal 

 electrical, chemical, and cohesive forces of the medium. The state 

 of matters thus required is, in fact, precisely realised by a symme- 

 trical arrangement of positive and negative atoms in the molecule, 

 such as the hexagonal molecule recently imagined by Lord Kelvin,* 

 and earlier by J. and P. Curie, to account for the piezo- electric quality 

 of quartz; the symmetry of the electric charges makes null the 

 aggregate electric moment and therefore the turning couple in an 

 electric field, while a differential polarity can still be developed under 

 strain of the crystal. 



According to the present theory of electrification, a discharge of 

 electricity from one conductor to another can only occur by the break- 

 ing down of the elasticity of the dielectric cether along some channel 

 connecting them ; and a similar rupture is required to explain the 

 transfer of an atomic charge to the electrode in the phenomenon of 

 electrolysis. We can conceive the polarisation increasing by the 

 accumulation of dissociated ions at the two electrodes of a voltameter, 

 until the stress in the portion of the medium between the ions and 

 the conducting plate breaks down, and a path of discharge is opened 

 from some ion to the plate. While this ion retained its charge, it 

 repelled its neighbours ; but now electric attraction will ensue, and 

 the one that gets into chemical contact with it first will be paired 

 with it by the chemical forces ; while if the conducting path to the 

 electrode remains open until this union is complete, the ion will 

 receive an opposite atomic charge from the electrode, which very con- 

 ceivably may have to be also of equal amount, in order to equalise 

 * Lord Kelvin, ' Phil. Mag.,' October, 1893. 



