632 M. Helmholtz on the Theory of Electro chjnamics, 



the cells there must be other balls, of invariable volume, as fric- 

 tion rollers. These would rotate freely; but their centres of 

 gravity, in insulating media, would merely be displaced by 

 elastic yielding of the cell-wall ; in conducting media, on the 

 contrary, at every displacement they must suffer a resistance 

 similar to friction in a viscous liquid. The transference of motion 

 between these balls would be effected only through the adhesion 

 of their surfaces to one another. Displacement of the last- 

 mentioned balls gives dielectric polarization of the medium ; 

 streaming of the same, an electric current ; rotation of the 

 elastic balls corresponds to the magnetizing of the medium, the 

 axis of rotation being the direction of the magnetic force. 



Now, although the idea of such a molecular structure of the 

 space-filling sether may be repugnant to our imagination as too 

 artificial, yet the hypothesis of Maxwell appears to me very im- 

 portant on this account — because it proves that there is nothing 

 in electrodynamic phenomena to compel us to attribute them to 

 an entirely anomalous sort of natural forces, to forces depending 

 not merely on the situation of the masses in question, but also 

 on their motion. Indeed, out of the assumption of those reac- 

 tions of the volume-elements of the sether upon each other 

 which Mr. Maxwell has assumed, a complete and mathematically 

 very elegant theory of all electric phenomena (magnetic, electro- 

 dynamic, and induction) can be developed; and the same theory 

 also gives an account of the phenomena of light. 



On the other hand, M. Weber's theory derives the explana- 

 tion of electrodynamic actions from distant forces of a peculiar 

 kind, acting between the points of the electrical masses, and de- 

 pending simultaneously on the distances and the relative motions 

 and accelerations of each pair of points. It gives comparatively 

 simple explanations of electrodynamic attractions and of the 

 induction-effects in linear conductors; and its analytic deduc- 

 tions accord perfectly, for all the phenomena to be observed in 

 closed linear currents, with the consequences of the potential- 

 law derived by F. E. Neumann from the phenomena. On this 

 account, Weber^s theory (which preceded MaxwelFs) was very 

 favourably received, especially by the German physicists. It 

 had, and moreover retains, decidedly the merit of every acute 

 and original thought which endeavours to strike out new paths 

 in science when the old ones appear to lead into an inextri- 

 cable thicket. I hardly need here remark that the value of such 

 an attempt, if it was suflacient for the state of knowledge at the 

 time, is not diminished when, after twenty-five years^ progress 

 of science, the impossibility is shown of carrying it out. Even 

 then such an attempt has not been fruitless. A reconnaissance 

 of unknown ground lying beside the road hitherto kept, if car- 



