SECT. XXVIII. ATTRACTION AND REPULSION. 283 



which the electric equilibrium is destroyed by friction ; then the 

 positive and negative electricities are called into action or sepa- 

 rated ; the positive is impelled in one direction, and the negative 

 in another. Electricities of the same kind repel, whereas those 

 of different kinds attract each other. The attractive power is 

 exactly equal to the repulsive power at equal distances, and when 

 not opposed they coalesce with great rapidity and violence, pro- 

 ducing the electric flash, explosion, and shock ; then the equili- 

 brium is restored. One kind of electricity cannot be evolved 

 without the evolution of an equal quantity of the opposite kind. 

 Thus when a glass rod is rubbed with a piece of silk, as much 

 positive electricity is elicited in the glass as there is negative in 

 the silk. The kind of electricity depends more upon the me- 

 chanical condition than on the nature of the surface ; for when 

 two plates of glass, one polished and the other rough, are rubbed 

 against each other, the polished surface acquires positive and the 

 rough negative electricity. The manner in which friction is 

 performed also alters the kind of electricity. Equal lengths of 

 black and white ribbon applied longitudinally to one another, 

 and drawn between the finger and thumb so as to rub their 

 surfaces together, become electric. When separated the white 

 ribbon is found to have acquired positive electricity, and the 

 black negative ; but if the whole length of the black ribbon be 

 drawn across the breadth of the white, the black will be posi- 

 tively and the white negatively electric when separated. The 

 friction of the rubber on the glass plate of the electrifying 

 machine produces abundance of static electricity. The friction 

 of the steam on the valve of an insulated locomotive steam- 

 engine produces seven times the quantity of electricity that an 

 electrifying machine would do with a plate three feet in diameter, 

 worked at the rate of 70 revolutions in a minute. Pressure 

 is a source of electricity which M. Becquerel has found to be 

 common to all bodies ; but it is necessary to separate them to 

 prevent the reunion of the electricities. When two substances 

 of any kind whatever are insulated and pressed together they 

 assume different electric states, but they only show contrary 

 electricities when one of them is a good conductor. When both 

 are good conductors they must be separated with extreme 

 rapidity to prevent a return to equilibrium. When the separa- 

 tion is very sudden the tension of the two electricities may be 



