• TO Pojlfcr'ipt to Volta's LelUrs 



terwards with each revolution of the machine ; the aiftlon of 

 which, according to the theory of eleftric atmofpheres, pro- 

 duces this cffe<St to the degree mentioned, and jullifies the 

 appellation of doublcr of eleftricity, which has been given to 

 this inftrument. 



I now come to the experiments, which (how that we are to 

 feek for the caufc which calls forth the action of the electric 

 fluid J which excites it, of whatever kind it be; determines 

 its tranfition^ &c. much rather in the mutual contact of the 

 metals, than in the contaft of the moift conductors with 

 thefe metals. Though, according to every circumilancej 

 we mufl; admit fome aftion of this kind in the latter contaft, 

 it cannot be denied that the former is certainly the moil ef- 

 feftual. At prefenl I fliall only mention the two following 

 experiments, which I contrived in fuch a manner that they 

 may ferve to explain a queftion of this kind, 



V. I left the two fixed plates of brafs without making any 

 alteration; took off the third moveable plate, and fupplied its 

 place by one of tin ; and arranged the machine in fuch a 

 manner that the latter ftood oppafite to one of the other 

 two plates. I theii applied to this tin plate a bit of brafs, 

 and to the oppafite fixed plate of brafs a piece of tin. M- 

 ter a convenient time, (for example an hour, v.hen the wea- 

 ther was perfectly dry,) I took away the two pieces of me- 

 tal, or only that of brafs, and made the moveable plate of 

 tin, which was i^ contact with the piece of brafs, to revolve 

 about thirty times. It then gave me very perceptible marks 

 pf pofitive eleftricity. 



VI. I reverfed the former experiment, and made the piece 

 of brafs touch the brafs plate, and the piece of tin the plate of 

 the fame metal. I, however, obtained nothing, or almofl: no- 

 thing ; even when the apparatus was left a much longer 

 time in that fituation, and when the machine had madft 

 twice or three times as many revolutions. 



Thefe two experiments are rcprefented by fig. 25 and 26 : 

 ^vhcre L is the piece of brafs: E that of tin; and a a the 



