Theories of Electricity 13^ 



without! though the plenum presses violently to expand, 

 and the hungry vacuum seems to attract as violently in 

 order to be filled." 



One cannot but admire the remarkable clearness of 

 this explanation of the then mysterious action of the 

 Leyden jar. In fact, with few alterations it would serve 

 the same purpose to-day. The fundamental conditions 

 to be fulfilled in charging and discharging the jar are 

 brought out with such emphasis and point that it is not 

 difficult to imagine Franklin, in the guise of a modern 

 professor of physics, expounding the action of the Ley- 

 den jar to a class-room of inattentive students — an image 

 that may appeal more forcibly to my colleague. Pro- 

 fessor Nichols, than to my audience. 



While the theory of Franklin of the action of the 

 Leyden jar was a notable advance in electrical ideas, we 

 can now see clearly the imperfections in his explanation. 

 The idea of electrical induction still remained to be fully 

 developed, and it was not till nearly a century later that 

 Faraday clearly laid down the true action of the glass 

 or dielectric. 



In a later letter in 1749, Franklin returns again to 

 his views of the nature of this electrical fluid: 



" The electrical matter consists of particles extremely 

 subtile, since it can permeate common matter, even the 

 densest metals, with such ease and freedom as not to 

 receive any perceptible resistance. 



