846 Prof. Maxwell on the Theory of Molecular Vortices 
and so to change their place, provided they keep within one 
complete molecule of the substance ; but in passing from one 
molecule to another they experience resistance, and generate 
irregular motions, which constitute heat. These particles, 3 in our 
theory, play the part of electricity. Their motion of translation 
constitutes an electric current, their rotation serves to transmit 
the motion of the vortices from one part of the field to another, 
and the tangential pressures thus called into play constitute elec- 
tromotive force. ‘The conception of a particle having its motion 
connected with that of a vortex by perfect rolling contact may 
appear somewhat awkward, I do not bring it forward as a mode 
of connexion existing in nature, or even as that which IT would 
willingly assent to as an electrical hypothesis, It is, however, a 
mode of connexion which is mechanically conce ivable, and easily 
investigated, and it serves to bring out the actual mechanical 
connexions between the known electro-magnetic phenomena; so 
that I venture to say that any one who understands the provisional 
and temporary character of this hypothesis, will find himself 
rather helped than hindered by it im his search after the true 
interpretation of the phenomena. 
The action between the vortices and the layers of particles is 
in part tangential; so that if there were any slipping or differen- 
tial motion between the parts in contact, there would be a loss of 
the energy belonging to the lines of force, and a gradual trans- 
formation of that energy into heat. Now we know that the 
lines of foree about a magnet are maintained for an indefinite 
time without any expenditure of energy ; so that we must con- 
clude that wherever there is tangential action between different 
parts of the medium, there is no motion of shpping between 
those parts. We must therefore conceive that the vortices and 
particles roll together without shpping; and that the interior 
strata of each vortex receive their proper velocities from the ex- 
terior stratum without slipping, that is, the angular velocity 
must be the same throughout e ach vortex 
The only process in whic th electro-mi enetic energy is lost and 
transformed into heat, is in the passage of electricity from one 
moleeule to another, In all other cases the energy of the vor- 
tices ean only be diminished when an equivalent quantity of me- 
chanical work is done by magnetic action. 
(6) The effect of an electric current upon the surrounding 
medium is to make the vortices in contact with the current 
revolve so that the parts next to the current move in the same 
direction as the current. The parts furthest from the current 
will move in the opposite direction ; and if the medium is a con- 
ductor of electricity, so that the particles are free to move in an 
direction, the particles touching the outside of these vorticeswil 
