46 Electric and Magnetic Science 



Franklin's own writings. It originated in connexion with the 

 explanation of the Ley den jar, a matter which is discussed 

 in his third letter to Collinson, of date September 1st, 1747. 

 In charging the jar, he says, a quantity of electricity is taken 

 away from one side of the glass, by means of the coating 

 in contact with it, and an equal quantity is communi- 

 cated to the other side, by means of the other coating. The 

 glass itself he supposes to be impermeable to the electric 

 fluid, so that the deficiency on the one side can permanently 

 coexist with the redundancy on the other, so long as the two 

 sides are not connected with each other ; but when a con- 

 nexion is set up, the distribution of fluid is equalized through 

 the body of the experimenter, who receives a shock. 



Compelled by this theory of the jar to regard glass as 

 impenetrable to electric effluvia, Franklin was nevertheless well 

 aware* that the interposition of a glass plate between an 

 electrified body and the objects of its attraction does not shield 

 the latter from the attractive influence. He was thus driven to 

 supposef that the surface of the glass which is nearest the 

 excited body is directly affected, and is able to exert an 

 influence through the glass on the opposite surface ; the latter 

 surface, which thus receives a kind of secondary or derived 

 excitement, is responsible for the electric effects beyond it. 



This idea harmonized admirably with the phenomena of 

 the jar ; for it was now possible to hold that the excess of 

 electricity on the inner face exercises a repellent action through 

 the substance of the glass, and so causes a deficiency on the 

 outer faces by driving away the electricity from it.J 



Franklin had thus arrived at what was really a theory of 

 action at a distance between the particles of the electric fluid ; 

 and this he was able to support by other experiments. " Thus," 

 he writes, " the stream of a fountain, naturally dense and con- 

 tinual, when electrified, will separate and spread in the form of 

 a brush, every drop endeavouring to recede from every other 



* New Experiments, 1750, 28. t Hid., 1750, 34. 



J Ibid., 1750, 32. Letter v. 



