VOL, LXVIII.] PHILOSOPHICAL TKANSACTIONS. 4/1 



cake ; and therefore the metal plate being removed, by the insulating handle, 

 from the cake, gives evident signs of having lost a part of its natural share of 

 electric fluid ; or, in other words, of being electrified negatively, the resinous 

 cake being more tenacious of the state of electricity, which it had acquired, than 

 the metal plate. If the resinous cake be in a state of negative electricity, 

 (which it acquires by a friction either with a dry hand, a piece of leather, or a 

 rough skin ; or by sliding the negative part of a charged phial on it, or by many 

 other ways) the contrary must happen, viz. the electric fluid of the metal plate, 

 finding a kind of vacuum on the resin cake, rushes upon it, and thus leaves its 

 opposite extremity in a negative case. 



A conducting body, having its natural quantity of electric fluid, bei.ig brought 

 near this metal plate, gives it a spark, which spark the metal plate retains as an 

 additional quantity. If the metal plate be afterwards separated from the cake, 

 it must retain this additional quantity which it has received from the approaching 

 body ; because the resinous cake being, from its nature, more tenacious of the 

 state of electricity acquired than the metal, remains thereabout in the same con- 

 dition as it was before the metal plate was placed on it ; but the metal plate, 

 having acquired an additional quantity in the time it was placed on the cake, car- 

 ries with it this quantity, and must therefore return from the cake in a positive 

 state. This confirms what was before said, that in the first case the cake of 

 resin does not quit readily the electric fluid which it had acquired ; and, in the 

 2d case, does not steal from the metal plate the electric fluid which it had lost. 

 What happens to the metal plate placed on the resinous cake, happens also to the 

 metal on which the resinous cake is commonly fixed ; but the reverse must take 

 place, that is, when the upper plate is taken off^ the cake in a positive state, the 

 metal under the cake must be found in a negative state, if the electrophore be 

 placed on an electrical stand. 



It may be asked, what difference there is between an electrophore and a 

 coated phial, or a flat glass coated on both sides and charged ? I answer, that 

 there is none at all, if both or only one of the metallic coatings can be taken off 

 by silk strings, or a piece of sealing wax, or any other insulating substance. 

 The very same day I received the electrophore sent to me by his royal highness 

 the archduke Ferdinand from Milan (which electrophore was a thin resinous 

 cake stuck on a flat piece of metal, to which was adapted a metal plate furnished 

 with a glass handle to lift it up) I produced the same appearance by a common 

 pane of glass and the metal plate of the electrophore ; but soon finding that 

 glass, however dry, quickly loses its electricity (probably from its easily attracting 

 moisture from the air) I tried to cover it with a resinous substance, or to varnish 

 it over with a hard copal varnish ; by which means it was easily excited by fric- 



