PREFACE. XXI 



" A theory of molecular vortices which I worked out at considerable length was 

 published in the Phil. Mag. for March, April and May, 1861, Jan. and Feb. 1862." 



" I think we have good evidence for the opinion that some phenomenon of rotation 

 is going on in the magnetic field, that this rotation is performed by a great number 

 of very small portions of matter, each rotating on its own axis, that axis being parallel 

 to the direction of the magnetic force, and that the rotations of these various vortices 

 are made to depend on one another by means of some mechanism between them." 



" The attempt which I then made to imagine a working model of this mechanism 

 must be taken for no more than it really is, a demonstration that mechanism may 

 be imagined capable of producing a connection mechanically equivalent to the actual 

 connection of the parts of the Electromagnetic Field." 



This paper is also important as containing the first hint of the Electromagnetic Theory 

 of Light which was to be more fully developed afterwards in his third great memoir 

 " On the Dynamical Theory of the Electromagnetic Field." This me/noir, which was presented 

 to the Royal Society on the 27th October, 1864, contains Maxwell's mature thoughts on a 

 subject which had so long occupied his mind. It was afterwards reproduced in his Treatise 

 with trifling modifications in the treatment of its parts, but without substantial changes 

 in its main features. In this paper Maxwell reverses the mode of treating electrical 

 phenomena adopted by previous mathematical writers ; for while they had sought to build 

 up the laws of the subject by starting from the principles discovered by Ampere, and 

 deducing the induction of currents from the conservation of energy, Maxwell adopts the 

 method of first arriving at the laws of induction and then deducing the mechanical 

 attractions and repulsions. 



After recalling the general phenomena of the mutual action of currents and magnets 

 and the induction produced in a circuit by any variation of the strength of the field in 

 which it lies, the propagation of light through a luminiferous medium, the properties of 

 dielectrics and other phenomena which point to a medium capable of transmitting force 

 and motion, he proceeds. 



" Thus then we are led to the conception of a complicated mechanism capable 

 of a vast variety of motions but at the same time so connected that the motion of 

 one part depends, according to definite relations, on the motion of other parts, these 

 motions being communicated by forces arising from the relative displacement of their 

 connected parts, in virtue of their elasticity. Such a mechanism must be subject 

 to the laws of Dynamics." 



On applying dynamical principles to such a connected system he attains certain general 

 propositions which, on being compared with the laws of induced currents, enable him to 

 identify certain features of the mechanism with properties of currents. The induction of 

 currents and their electromagnetic attraction are thus explained and connected. 



