Possible explanations of specific effect of the dielectric 175 



incompressible fluid, and consequently the charge of the plate of air in 

 these experiments ought to bear a greater proportion to that of the globe 

 than if they had been connected to the vial by which they were electrified 

 by canals of incompressible fluid. 



349] It was said in Art. 339 that the charges of the glass plates were 

 rather more than eight times greater than they ought to be by the theory, 

 if the electric fluid did not penetrate to any sensible depth into the glass. 

 Though this is what I did not expect before. I made the experiment, yet 

 it will agree very well with the theory if we suppose that the electricity, 

 instead of entering into the glass to an extremely small depth, as I thought 

 most likely when I wrote the second part of this work*, is in reality able 

 to enter into the glass to the depth of -^ of the whole thickness of the glass, 

 that is, to such a depth that the space into which it can not penetrate is 

 only of the thickness of the glass, as in that case it is evident that the 

 charge should be as great as it would be if the thickness of the glass was 

 only | of its real thickness, and the electricity was unable to penetrate 

 into it at all. 



350] There is also a way of accounting for it without supposing the 

 electricity to enter to any sensible depth into the glass, by supposing that 

 the electricity at a certain depth within the glass is moveable, or can 

 move freely from one side of the glass to the other. 



Thus, in Fig. 25, let ABDE be a section of the glass plate perpen- 

 dicular to its plane, suppose that the electricity from without can pene- 



Fig. 25. 



trate freely into the glass as far as the line db or ed but not further, 

 suppose too that within the spaces abfla and edSe the electric fluid is 

 immoveable, but that within the space a/?8e it is moveable, or is able to 

 move freely from the line aft to 8e. Then will the charge of the plate be 

 just the same as on the former supposition, provided the distances aa 

 and ee are each -fa of the thickness of the plate f. 



* [Refers to Art. 132.] 



| The only reason why I suppose the electric fluid to be able to enter into the 

 glass from without as far as the lines ab and ed is that Dr Franklin has shewn that 

 the charge resides chiefly in the plate of glass and not in the coating, and conse- 

 quently that the electricity is able to penetrate into the glass to a certain depth. 

 Otherwise it would have done as well if we had supposed the fluid to be immoveable 

 in the whole spaces ABfta and ED8e, and that the distance A a and Et are each 



of AE. 



