SECT. XXVIII. NON-ELECTRICS. 273 



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 ; or in other words there is a redundancy in the 

 glass and a proportional deficiency in the silk. The 

 kind of electricity depends more upon the mechanical 

 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 ac- 

 quires positive and the rough negative electricity ; that 

 is, the one gains and the other loses. The manner in 

 which friction is performed also alters the kind of elec- 

 tricity. Equal lengths of black and white riband ap- 

 plied longitudinally to one another, and drawn between 

 the finger and thumb, so as to rub their surfaces to- 

 gether, become electric. When separated, the white 

 riband is found to have acquired positive electricity, and 

 the black has lost it, or become negative : but if the 

 whole length of the black riband be drawn across the 

 breadth of the white, the black will be positively and 

 the white negatively electric when separate. Elec- 

 tricity may be transferred from one body to another in 

 the same manner as heat is communicated, and like it 

 too, the body loses by the transmission. Although' no 

 substance is altogether impervious to the electric fluid, 

 nor is there any that does not oppose some resistance 

 to its passage, yet it moves with much more facility 

 through a certain class of substances called conductors, 

 such as metals, water, the human body, &c., than 

 through atmospheric air, glass, silk, &c., which are 

 therefore called non-conductors. The conducing power 

 is affected both by temperature and moisture.^ 



Bodies surrounded with non-conductors are said to be 

 insulated, because, when charged, the electricity cannot 

 escape. When that is not the case, the electricity is 

 conveyed to the earth, which is formed of conducting 

 matter; consequently it is impossible to accumulate 

 electricity in a conducting substance that is not insu- 

 lated. There are a great many substances called non- 

 electrics, in which electricity is not sensibly developed 

 by friction, unless they be insulated, probably because it 

 is carried off by their conducting power as soon as 

 elicited. Metals, for example, which are said to be 

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