CONTRIBUTIONS TO SCIENCE. 541 



move, and the substance of the cell therefore undergoes a 

 shearing strain which is resisted by its elasticity, and the 

 state of strain of the cells is propagated through the dielectric 

 by means of the displacement of the electric particles which 

 behave like perfectly incompressible bodies. When the 

 force producing the original displacement is removed the 

 cells resume their original form in virtue of their elasticity, 

 the electric particles return to their normal positions, and 

 the energy of the strained elastic cells expends itself in the 

 work done during the electric discharge. Thus the same 

 medium which serves as the vehicle of magnetic force and 

 produces all the phenomena of electromagnetism also serves 

 for the transmission of the force between charges of statical 

 electricity and as a reservoir of the energy due to electro- 

 static charges. If the dielectric be divided into cells by 

 unit tubes of force and equipotential surfaces drawn for every 

 unit difference of potential, each cell will contain the same 

 amount of energy. 1 The following quotations from the 

 paper in the Philosophical Magazine explain the application 

 of the hypothesis of molecular vortices to statical electricity 

 in Maxwell's own words : 



According to our theory the particles which form the parti- 

 tions between the cells constitute the matter of electricity. The 

 motion of these particles constitutes an electric current ; the 

 tangential force with which the particles are pressed by the 

 matter of the cells is electromotive force, and the pressure of 

 the particles on each other corresponds to the tension or poten- 

 tial of the electricity. 



A conducting body may be compared to a porous membrane 

 which opposes more or less resistance to the passage of a fluid; 

 while a dielectric is like an elastic membrane, which may be im- 

 pervious to the fluid, but transmits the pressure on the one side 

 to [the fluid] on the other. 



In a dielectric under induction, we may conceive that the 

 electricity in each molecule is so displaced that one side is 



1 See Elementary Treatise on Electricity by Professor James Clerk 

 Maxwell, published by the Clarendon Press, 1881. 



