On the PhOTiomena of the Eledrophoms. 293 



These experiments arc the result of my own observation, 

 and, I flatter niyselF, may be depended upon : thw have all 

 been repeated more than once; and indeed a considerable 

 degree of care is requisite in ascertaining the kind of elec- 

 tricity indicated by the diviergency of pith balls, as a stronger 

 electricity in glass or wax will frequently attract a weaker 

 of the sanif! so'i-t in these light bodies. I have found it most 

 can\^eHi£Ut to apply an excited stick of wax and glass alter- 

 nately to the plate at some distance from the electrometer, 

 and never to rest satisfied till both of these tests concurred 

 in their results. By the use of two similar electrometers I 

 have had great advantage in perceiving the changes produced 

 in each of the metallic parts : and perhaps it may be in 

 some measure owing to the employn)efit of only one elec- 

 trometer in experiments upon this instrument that so much 

 uncertainty has prevaiJed, as I do not -recollect to have met 

 with any accouni where the state of sole and covej wete 

 ascertained by the same means and at the same time. 



I now proceed to offer a general view of the observation* 

 T have met with on this subject. 



The general result given by Adaiias (Essay on Electricity, 

 p. 338,) is thefollowmg: 



*• By exaujining with pith balls it appears, 

 " 1. That the cover acquires a weak, posit-ive electricity 

 u-hen imposed on excited cake. (See preceding experiment, 

 No. 7.) 



*' 2. That when the cover is touched by the finger it 

 loses alJ its electricity. (See No. 17.) 



" 3. When the cover is touched by the finger, and re- 

 moved from the cake, it becomes strongly positive. (See- 

 No. g.) 



In ofdcr to account f»r these appearances he oSfers the 

 following considerations : 



*' The cake may be conceived to consist of sev-eral hori- 

 zontal strata, so that the upper stratum, when excited, is 

 insulated by the inferior strata. Now, insulated electrics 

 produce the opposite electricity on bodies brought within 

 the sphere of their xiction ; i. o. insulated and excited glass 

 »rodui-es negative, and wax positive, electricity an other 

 insulated bodies : therefore the surface of the t.'le,ctrophorus 

 take sliMuld proflucc positive electricity in the cover, eon- 

 formablv to experience." But as it is not very obvious how 

 the electricity should become stronger by removal out of the 

 sphere of action of the excited electric, he adds : " electric 

 bodies do not put the fluid in that degree of motion which 

 la necessary lo produce the spark, or exhibit the phaunonicju. 

 Tji of 



