422 HYPOTHESIS OF TWO FLUIDS. 



bodies exhibit positive or negative electricity 

 according to their colour, or the condition of 

 their surface that smooth glass is rendered posi- 

 tive by friction with woollen cloth, and negative 

 when made rough by grinding it with emery, 

 all other things being equal.* 



It cannot be denied that the electricity of a 

 common machine is derived chiefly from the 

 rubber ; for it is well known that its quantity is 

 greatly increased by spreading over the rubber 

 an amalgam of zinc or tin, the oxidation of 

 which supplies electricity freely, as in the action 

 of a voltaic battery ; while amalgams of silver 

 and platina, which do not oxidize at ordinary 

 temperatures, are of no use. But is it not evi- 

 dent, that if the prime conductor receive its 

 electricity from the rubber, that positive and 

 negative are only different proportions of the 

 same fluid? that it accumulates and becomes 

 plus on the prime conductor, because of its 

 greater attraction for it?| and that whether 

 positive or negative, its essential properties are 



* Is not the attraction of rough glass for electricity less than 

 that of smooth glass, for the same reason that caloric escapes 

 more rapidly from rough than from smooth metals ? and does it 

 not accumulate upon insulated metals in greater quantities than 

 on electrics for the same reason that the former are good con- 

 ductors? (See page 194.) 



f Something analogous takes place during the action of the 

 Galvanic pile. When the battery is composed of zinc and silver, 



