XX PREFACE. 



Electricity. His object, he says, is to find a physical analogy which shall help the mind 

 to grasp the results of previous investigations "without being committed to any theory 

 founded on the physical science from which that conception is borrowed, so that it is neither 

 drawn aside from the subject in the pursuit of analytical subtleties nor carried beyond the 

 truth by a favorite hypothesis." 



The laws of electricity are therefore compared with the properties of an incompressible 

 fluid the motion of which is retarded by a force proportional to the velocity, and the fluid 

 is supposed to possess no inertia. He shews the analogy which the lines of flow of such 

 a fluid would have with the lines of force, and deduces not merely the laws of Statical 

 Electricity in a single medium but also a method of representing what takes place when the 

 action passes from one dielectric into another. 



In the latter part of the paper he proceeds to consider the phenomena of Electro- 

 magnetism and shews how the laws discovered by Ampere lead to conclusions identical with 

 those of Faraday. In this paper three expressions are introduced which he identifies with 

 the components of Faraday's electrotonic state, though the author admits that he has not 

 been able to frame a physical theory which would give a clear mental picture of the 

 various connections expressed by the equations. 



Altogether this paper is most important for the light which it throws on the principles 

 which guided Maxwell at the outset of his electrical work. The idea of the electrotonic 

 state had already taken a firm hold of his mind though as yet he had formed no physical 

 explanation of it. In the paper "On Physical Lines of Force" printed in the Philosophical 

 Magaziiie, Vol. XXI. he resumes his speculations. He explains that in his former paper he 

 had found the geometrical significance of the Electrotonic state but that he now proposes 

 "to examine magnetic phenomena from a mechanical point of view." Accordingly he propounds 

 his remarkable speculation as to the magnetic field being occupied by molecular vortices, 

 the axes of which coincide with the lines of force. The cells within which these vortices 

 rotate are supposed to be separated by layers of particles which serve the double purpose 

 of transmitting motion from one cell to another and by their own motions constituting ;in 

 electric current This theory, the parent of several working models which have been devised 

 to represent the motions of the dielectric, is remarkable for the detail with which it is 

 worked out and made to explain the various laws not only of magnetic and electromagnetic 

 action, but also the various forms of electrostatic action. As Maxwell subsequently gave a 

 more general theory of the Electromagnetic Field, it may be inferred that he did not desire 

 it to be supposed that he adhered to the views set forth in this paper in every particular ; 

 but there is no doubt that in some of its main features, especially the existence of 

 rotation round the lines of magnetic force, it expressed his permanent convictions. In his 

 treatise on " Electricity and Magnetism," Vol. II. p. 416, (2nd edition 427) after quoting from 

 Sir \V. Thomson on the explanation of the magnetic rotation of the plane of the polarisitinn 

 of light, he goes on to say of the present paper, 



