224 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



to diverge the electroscope forty degrees. Now this divergence remained when at 

 least il-ths of the air was withdrawn from the receiver. In this state of the exhaustion 

 a similar ball, h\ in a neutral state, was made to approach the former by means of a 

 similar sliding-rod and collar fixed in the side of the receiver : as the ball h' ap- 

 proached, the electroscope began to collapse, and agair/ opened as it was withdrawn, 

 so that at the point of contact, the divergence was permanently diminished. 



Since the atmospheric particles in this experiment were to a great extent with- 

 drawn, without any change being indicated by the electroscope, — whilst, on the con- 

 trary, its divergence became instantly decreased, and again restored on withdrawing 

 the neutral ball h\ or otherwise permanently diminished on contact, — we may con- 

 clude, that the atmospheric influence was indefinitely small in respect of the indica- 

 tions of the instrument ; and that the subsequent collapsing of the electroscope was 

 occasioned by causes altogether connected with the metallic bodies themselves. 



(g.) An excited gold-leaf electroscope, c, fig. 12, inclosed in an airtight bulb of 

 glass so as to prevent any escape of the contained air, was placed on an insulated rod, 

 and coveped by a large receiver : the divergence remained unchanged when ^-§^ths of 

 the air was withdrawn. On approaching an insulated ball, n, to the cap of the instru- 

 ment, which also terminated in a large sphere, c, the leaves gradually closed*. 



28. The decreased intensity observable by the electrometer (16.) may be referred 

 therefore, partly, to the change of density of the electrical stratum arising from the 

 diminished quantity in any given point, and partly, to the influence of the electrified 

 substance itself, by which a portion of the force on external bodies becomes more or 

 less masked, or controlled. 



29. The conditions of the controlled action, in cases of electrical accumulation on 

 coated glass, are precisely the same aj those above mentioned (28.). A coated jar 

 may be considered as a species of compound conductor, in which the controlling 

 effect of the insulated coating in respect of the electrometer is greatly increased by 

 its proximity to the other in a free state ; hence a much greater quantity may be ac- 

 cumulated on a given extent of surface with the same intensity. The difference, there- 

 fore, between electrical accumulation on coated glass and that on simple conductors 

 is only in degree of effect ; the laws incidental to the electrified substance remain the 

 same. 



30. We may infer on the principles above exposed (21.), that the controlling force 

 of bodies when electrified, in respect of the action exerted upon their electricity by 

 those which are neutral, would continually decrease as the quantity accumulated on 

 a given point increases, so that at last, by the superior force of the neutral body, it 

 would become nothing, or very nearly so ; hence a discharge ensues, for the force in the 



* The facility with which electrified bodies retain their charge in rarefied air, under perfect insulation, and 

 when removed sufficiently from the influence of neutral conducting substances, is somewhat at variance with 

 the elastic hypothesis of electricity as generally understood. Having been at first led to adopt this hypothesis 

 in all its generality, I was not prepared for such a result. 



