Mr. Laming o?i the primary Forces of Eleclricity. 4t5 



in plenum, to another supposed to be insulated in an absolute 

 vacuum^ is equal to nothing. 



33. From this it evidently follows, first, that when a con- 

 ductor containing a plus charge be partly inclosed in a re- 

 ceiver, and the air within the latter is gradually withdrawn, 

 the free electricity will progressively recede to those parts of 

 the conductor which are external to the receiver; for by the 

 theory the several parts of a conductor can never differ in 

 intensity (28.); which in this case they would do unless the 

 charge requiring compensation, and the air by which it is 

 compensated {both within the receiver) diminished in the same 

 ratio. 



34'. In the second place we learn that by reducing the 

 density of the air when a charged conductor is insulated 

 wholly within the receiver, we virtually increase the quantity 

 of electricity to be compensated by an assumed unit of air; 

 thus causing the force of attraction between that electricity 

 and its compensator to be increased. Now the intensities 

 being as the square of the quantities directly, and the quan- 

 tities virtually as the densities of the air directly, the electrical 

 intensity of any given chiarge is as the square of the atmo- 

 spherical density inversely ; and this is the conclusion Mr. 

 Harris has arrived at by experimenting in various ways*. 



35. We are thus enabled to understand why a conducting 

 body highly charged and insulated under a receiver gradually 

 becomes discharged as the vacuum becomes more perfect ; a 

 fact which has been long known, but until now never ex- 

 plained. It is usual to ascribe it to diminished resistance by 

 the atmospheric air ; but the phaenomenon may be made to 

 take place equally when the pressure remains undiminished ; 

 which has been proved by first abstracting part of the air 

 from a receiver and then heating the residuum ; under such 

 circumstances it was found that, however the temperature, 

 and with it of course the pressure, might be changed, so long 

 as the quantity of free electricity remained constant, its inten- 

 sity varied as the square of the density or quantity of the air, 

 inverselyf. 



36. The phaenomena which characterize the deflection of 

 the pith-balls, straws, gold-leaves, or other moveable parts of 

 diverging electroscopes, are rendered intelligible by the ap- 

 plication of our principles. Suppose the compensation of a 

 plus body B, free to move, to devolve entirely on two other 

 uninsulated fixed bodies, A A', placed on either side and at 

 equal distances from it ; B will have an equal tendency to 



* Phil. Trans. 1834, part ii. 



t Ibid., p. 228. 



