vi 



exclusive of treatises, is a long one, numbering over 100 papers, many 

 of which contain speculations of a profound character, worked out 

 with elaborate details of calculation. They treat of a variety of 

 subjects, the most important of which are — (1.) Electricity and Mag- 

 netism ; (2.) The Kinetic Theory of Gases; (3.) Colour Perception ; 

 (4.) Dynamics, including Astronomical Physics ; (5.) Elasticity ; 

 (6.) Optics. The two first named attracted more of his attention than 

 the others, and his writings on them form a sort of continuous series 

 in which we can follow the history of his ideas so as almost to trace 

 their gradual development. 



Thus, his first memoir on Electricity, entitled "On Faraday's Lines 

 of Force," possesses an interest apart from its intrinsic value, contain- 

 ing as it does the germs of the theories and methods which reached 

 their full growth in his great treatise on Electricity and Magnetism. 

 This memoir is in two parts, differing in object and treatment. Whilst 

 the first half aims at a vivid representation of Faraday's conception 

 of lines of electric and magnetic force, the second is a mathematical 

 exposition of what Faraday calls the electrotonic state of bodies, and 

 an analytical investigation, based on Faraday's laws, of the electro- 

 motive forces acting on a conductor due to the motion of magnets or 

 currents of electricity outside of it. 



Faraday's doctrine that electric and magnetic effects are conveyed 

 by a medium and not by action at a distance, found in Maxwell an 

 ardent believer, who set himself the task of searching out by what 

 kind of mechanism this is accomplished. His first attempt at an 

 explanation is contained in a series of papers in the " Philosophical 

 Magazine," 1861-62. Beginning with magnetic phenomena, he points 

 out that a medium transmitting magnetic action must be under a 

 stress in which there is excess of pressure in all directions perpen- 

 dicular to the lines of force, in other words, a stress consisting of a 

 tension in the direction of the line of force combined with a hydro- 

 static pressure. A stress of this character would be produced by a 

 system of molecular vortices, the axes of which are in the direction 

 of the lines of force. A similar representation may be made of the 

 stress due to the magnetic action of electric currents. 



Taking, then a system of such vortices, he finds that the most general 

 form of the expressions for the force components, due to the vortices at 

 any point of the medium, is identical with that which would arise from 

 magnets and electric currents. In the course of the work the magnetic 

 force is identified with the velocity of the vortex, and the coefficient of 

 magnetic induction is it times the density of the medium, or fji—irp. 

 The chief difficulty is in the geometrical conception of the motion of 

 the vortices, two parallel vortices moving in opposite directions in 

 those parts where they are contiguous. Maxwell gets over this 

 difficulty by supposing that the vortices are separated by layers of 



