180 Discussion of results on spreading of charge 



was prevented from spreading on the surfaces as it should seem by these 

 experiments, or whether it was not rather owing partly to the error of 

 the experiment, and partly to there not being so much difference in the 

 distance to which the electricity spreads on the surface of the glass ac- 

 cording to the different degree in which it is electrified, as I imagined. 



If the first of these suppositions is true, I do not know how to reconcile 

 it with the theory, except by supposing that the greater the force with 

 which the plate is electrified the less is the depth to which the electricity 

 penetrates into the glass, or the less is the thickness of the spaces in which 

 we supposed the fluid to be moveable. 



Though it seemed natural to expect that the electric fluid should 

 penetrate further into the glass, or that the fluid within the glass should 

 move through a greater space when the glass was strongly electrified than 

 when weakly, that is, when the force with which the fluid was impelled 

 was great than when it was small, yet it is not strange that it should be 

 otherwise, as it is very possible that the electric fluid may penetrate with 

 great freedom to a certain depth within the glass, and that no ordinary 

 force shall be able to impel it sensibly further, and in like manner it is 

 very possible that the fluid may be able to move with perfect ease in the 

 space ae (Fig. 25) and yet that no ordinary force shall be able to move 

 the fluid at all beyond that space. 



But it would be very strange that the fluid should penetrate to a less 

 depth within the glass, or that the fluid within the glass should move 

 through a less space when the glass is strongly electrified than when weakly. 



363] The reader perhaps may be tempted from this circumstance to 

 think that the reason of the actual charge of the glass plates so much 

 exceeding their computed charge is not owing to the electric fluid pene- 

 trating into the glass, or to any motion of the fluid within the glass, but 

 to some error in the theory. But I think the experiments on the plate 

 of air [Art. 344] form a strong argument in favour of its being owing to 

 the penetration of the electric fluid into, or its motion within the glass, 

 for it appears plainly from these experiments that the electric fluid does 

 not penetrate into the air, and on account of the fluidity of the air it 

 seems very improbable that the electric fluid within the air should be 

 able to move in the manner we supposed it to do within the glass ; whereas 

 it appears plainly from Dr Franklin's analysis of the Leyden vial, that 

 the electric fluid does actually penetrate into the glass. 



Therefore as this excess of the observed charge above the computed 

 does not take place in the plate of air, where it could not do it consistently 

 with the theory, but does in the glass plate, where it may do so consistently 

 with the theory, I think there seems great reason to think that it is not 

 owing to any defect in the theory, but to some such motion of the electricity 

 as we have supposed. 



