176 Specific effect of the dielectric 



351] But I think the most probable supposition is that there are a 

 great number of spaces within the thickness of the glass in which the 

 fluid is alternately moveable and immoveable. 



d 



Fig. 26. 



Thus let ABDE (Fig. 26) represent a section of the plate of glass as 

 before, and let the glass be divided into a great number of spaces by the 

 parallel lines ab, a/3, ed, eS, &c., and suppose that in the two outermost 

 spaces ABba and EDde the fluid is moveable, that in the two next spaces 

 ab^a and ed8e it is immoveable, and that in the two next spaces it is 

 moveable, and so on. The charge will be the same as before, supposing 

 the sum of the thickness of the spaces in which the electricity is immove- 

 able to be % of the whole thickness of the glass, as it is shewn that the 

 charge of such a plate will be the same as that of a plate in which the 

 electricity is entirely immoveable, whose thickness is equal to the sum of 

 the thicknesses of those spaces in which we supposed the fluid immove- 

 able*. 



352] It must be observed that in those spaces in which we supposed 

 the fluid to be moveable, as in the space ABba for example, though the 

 fluid is able to move freely from the plane Ab to ab, that is, though it 

 moves freely in the direction Aa or aA, or in a direction perpendicular 

 to the plane of the plate, yet it must not [be] able to move lengthways, 

 or from A to B, for if it could, and one end of the plate AE was electrified, 

 some fluid would instantly flow from AE to BD, and make that end 

 overcharged, which is well known not to be the case. The same thing must 

 be observed also with regard to the two former ways of explaining this 

 phenomenon. 



353] The chief reason which induces me to prefer the latter way of 

 accounting for it is that in the two former ways the thickness of the 

 spaces in which the fluid is moveable must necessarily be very considerable. 

 In thick glass, for example, in a plate of the same thickness as D, it must 

 be not less than $$ of an inch in the first way of explaining it, and in 

 the second way it must be still greater. Now if the electric fluid is able 

 to move through so great a space in the direction AE, it seems extra- 

 ordinary that it should not be able to move in the direction AB, whereas 

 in the latter way of accounting for it the thickness of the spaces in which 

 the electricity is moveable may be supposed infinitely small, and conse- 



* [Prop. XXXV, Art. 169, and Note 15.] 



