90 . Prof. Maxwell on the Theory of Molecular Vortices 



and diamagnetism, and does not require us to admit cither M. 

 Weber's theory of the mutual action of electric particles in 

 motion, or our theory of cells and cell-walls. 



I am inclined to believe that iron differs from other substances 

 in the manner of its action as well as in the intensity of its mag- 

 netism; and I think its behaviour may be explained on our 

 hypothesis of molecular vortices, by supposing that the particles 

 of the iron itself are set in rotation by the tangential action of 

 the vortices, in an opposite direction to their own. These large 

 heavy particles would thus be revolving exactly as we have sup- 

 posed the infinitely small particles constituting electricity to re- 

 volve, but without being free like them to change their place and 

 form currents. 



The whole energy of rotation of the magnetized field would 

 thus be greatly increased, as we know it to be ; but the angular 

 momentum of the iron particles would be opposite to that of the 

 setherial cells and immensely greater, so that the total angular 

 momentum of the substance will be in the direction of rotation 

 of the iron, or the reverse of that of the vortices. Since, howr 

 ever, the angular momentum depends on the absolute size" of the 

 revolving portions of the substance, it may depend on the state 

 of aggregation or chemical arrangement of the elements, as well 

 as on the ultimate nature of the components of the substance. 

 Other phenomena in nature seem to lead to the conclusion that 

 all substances are made up of a number of parts, finite in size, 

 the particles composing these parts being themselves capable of 

 internal motion. 



Prop. XVIII. — To find the angular momentum of a vortex. 



The angular momentum of any material system about an axis 

 is the sum of the products of the mass, dm, of each particle multi- 

 plied by twice the area it describes about that axis in unit of 

 time ; or if A is the angular momentum about the axis of x, 



A=2^( y J-,|). 



As we do not know the distribution of density within the 

 vortex, we shall determine the relation between the angular 

 momentum and the energy of the vortex which was found in 

 Prop. VI. 



Since the time of revolution is the same throughout the vortex, 



the mean angular velocity a> will be uniform and = -, where a is 

 the velocity at the circumference, and r the radius. Then 

 A=^dmr^co P 



