angles to these lines, the numerical magnitude of the pressure being equal to that of the tension, and both varying as the square of the resultant force at the point.
In another place Maxwell argues that the state of stress described above is the only one consistent with the observed mechanical action of the electrified bodies and also with the observed equilibrium of the fluid dielectric which surrounds them.
Sir J. J. Thomson, who edited the third edition of Maxwell's treatise, takes exception to this claim. He says in a foot-note on page 165:
The subject of the stress in the medium will be further considered in the supplementary volume; it may however be noticed here that the problem of finding a system of stresses which will produce the same forces as those existing in the electric field is one which has an infinite number of solutions. That adopted by Maxwell is one which could not in general be produced by strains in an elastic solid.
This, in connection with the preceding quotation from Max well, indicates that Maxwell regarded his dielectric medium as necessarily a fluid; hence when the only dielectric between the positive and negative electrical condition is the ether of space, this medium must be a fluid. This seems to contradict the well-known fact that the only known forms of ether radiation are of the nature of transverse waves.
Maxwell goes no further than Faraday in explaining the condition of stress which is supposed to constitute induction. He merely attempts to describe it. He says:
It must be carefully borne in mind that we have made only one step in the theory of the action of the medium. We have supposed it to be in a state of stress, but we have not in any way accounted for this stress, or explained how it is maintained. ............. I have not been able to make the next step, namely, to account by mechanical considerations for these stresses in the dielectric. I therefore leave the theory at this point, merely stating what are the other parts of the phenomenon of induction in dielectrics.
Maxwell's claim is, accordingly, that if the dielectric medium between two charges, said charges being always necessarily upon the opposite surfaces of the dielectric, should contract in the direction of the lines of force normal to its charged boundaries and should expand in all directions at right angles to these lines of force, this contraction and expansion would enable him to account for the other phenomena of electrostatic induction.
Maxwell does make the further assumption that this stress in the dielectric is analogous to an elastic stress in material bodies. Thus he says:
