309 On th&-Phcenomeita pf the Eleciropkori/s. 



" Let a cushion be prepared with a metallic covering 

 at the back, and insulated by a glass stem ; then excite the 

 cylinder by means of this cushion, pursuant to tlie common 

 method : no electrical signs will be discovered while the 

 insulation is complete. Stop the motion of the cylinder, 

 placing the finger in contact with the cushion ; then remove 

 the cushion trom the excited cylinder, and it will give a 

 fspark : return it to the cylinder, and on removal it will re- 

 peatedly give a spark without any additional friction, at- 

 tended by tlie same circumstances as have been described 

 to take place in the electrophorus : now, by substituting a 

 glass plate for the cylinder, and supposing the cushion to 

 cover that plate, you form an electropliorus, without one 

 circumstance of diversity from tb.e common cylinder. 

 Again, if after exciting the electrophorus you ren)ove th..; 

 cover and apply a number of metallic points connected with 

 the ground to the excited surface, and afterwards replace 

 the cover, no more sparks can be obtained without a fresh 

 excitation. This is precisely similar to what is observed iii 

 working the cylinder ; for as soon as the excited part passes 

 the cushion, and is exposed to the action of a multitude of 

 pcints, it is rendered incapable of producing electrical signs 

 till it again comes into contact with the cushion." 



It is obvious that the phaenomena described, both with 

 respect to charges and excited electrics, bear a striking si- 

 milarity (as indeed might be expected) to those of the elec- 

 trophorus. We will now endeavour to explain the process- 

 detailed in the tabk? of phaenomena. 



I agree with Mr. Morgan that the general principle to 

 wliich these phicnomena ought to be referred is an increase 

 or diminution of attractive force between the particles of 

 bodies and particles of the electric lluid; or, in other words, 

 an alteration of the capacity oH bodies for the electric Huid, 

 in consequence either of excitation or contact with an ex- 

 cited substance. If I were asked for the cause of this 

 change of capacity, I should freelv c<.mfess my ignorance; 

 .for I cannot easily suppose a chcmicol change to be prp- 

 (luced, or any new arrangement of the parts created, by the 

 mere contact of bodies. It will be necessary to recollect, 

 what I believe was satisfactorily shown in my former paper, 

 that the divergence of pith balls ran only be depended upon 

 as indicating a disposition to receive or to part with a por- 

 tion of the electric fluid, or perhaps more properly an in- 

 creased or diminished attraction for it, and ought not to 

 be considered as any criterion of tlie absolute capacity of 

 bodies. 



When 



