86 Prof. Maxwell on the Theory of Molecular Vortices 



conceives magnetism to consist in currents of a fluid whose direc- 

 tion corresponds with that of the lines of magnetic force ; and 

 electric currents, on this theory, are accompanied by, if not de- 

 pendent on, a rotatory motion of the fluid about the axes of the 

 current. Professor Helmholtz* has investigated the motion of 

 an incompressible fluid, and has conceived lines drawn so as to 

 correspond at every point with the instantaneous axis of rotation 

 of the fluid there. He has pointed out that the lines of fluid 

 motion are arranged according to the same laws with respect to 

 the lines of rotation, as those by which the lines of magnetic force 

 are arranged with respect to electric currents. On the other 

 hand, in this paper I have regarded magnetism as a phenome- 

 non of rotation, and electric currents as consisting of the actual 

 translation of particles, thus assuming the inverse of the relation 

 between the two sets of phenomena. 



Now it seems natural to suppose that all the direct effects of any 

 cause which is itself of a longitudinal character, must be them- 

 selves longitudinal, and that the direct effects of a rotatory cause 

 must be themselves rotatory. A motion of translation along an 

 axis cannot produce a rotation about that axis unless it meets 

 with some special mechanism, like that of a screw, which con- 

 nects a motion in a given direction along the axis with a rotation 

 in a given direction round it ; and a motion of rotation, though 

 it may produce tension along the axis, cannot of itself produce a 

 current in one direction along the axis rather than the other. 



Electric currents are known to produce effects of transference 

 in the direction of the current. They transfer the electrical 

 state from one body to another, and they transfer the elements 

 of electrolytes in opposite directions, but they do notf cause the 

 plane of polarization of light to rotate when the light traverses 

 the axis of the current. 



On the other hand, the magnetic state is not characterized by 

 any strictly longitudinal phenomenon. The north and south 

 poles differ only in their names, and these names might be 

 exchanged without altering the statement of any magnetic pheno- 

 menon ; whereas the positive and negative poles of a battery are 

 completely distinguished by the different elements of water which 

 are evolved there. The magnetic state, however, is characterized 

 by a well-marked rotatory phenomenon discovered by Faraday J — 

 the rotation of the plane of polarized light when transmitted 

 along the lines of magnetic force. 



When a transparent diamagnetic substance has a ray of plane- 

 polarized light passed through it, and if lines of magnetic force 



* Crelle, Journal, vol. lv. (1858) p. 25. 



t Faraday, < Experimental Researches,' 951-954, and 2216-2220. 



+ Ibid., Series XIX. 



