620 LECTURE LIV. 



parts of the body, in particular to the chest, produces an agitation of the 

 nerves, which is not wholly unlike the effect of a weak electricity. 



The principal modes, in which the electric equilibrium is primarily 

 destroyed, are simple contact, friction, a change of the form of aggrega- 

 tion, and chemical combinations and decompositions. The electricity pro- 

 duced by the simple contact of any two substances is extremely weak, and 

 can only be detected by very delicate experiments : in general it appears 

 that the substance which conducts the more readily, acquires a slight 

 degree of negative electricity, while the other substance is positively 

 electrified in an equal degree. The same disposition of the fluid is also 

 usually produced by friction, the one substance always losing as much as 

 the other gains ; and commonly although not always, the worst conductor 

 becomes positive. At the instant in which the friction is applied, the 

 capacities or attractions of the bodies for electricity appear to be altered, 

 and a greater or less quantity is required for saturating them ; and upon 

 the cessation of the temporary change, this redundancy or deficiency is 

 rendered sensible. When two substances of the same kind are rubbed 

 together, the smaller or the rougher becomes negatively electrified ; perhaps 

 because the smaller surface is more heated, in consequence of its under- 

 going more friction than an equal portion of the larger, and hence becomes 

 a better conductor ; and because the rougher is in itself a better conductor 

 than the smoother, and may possibly have its conducting powers increased 

 by the greater agitation of its parts which the friction produces. The back 

 of a live cat becomes positively electrified, with whatever substance it 

 is rubbed ; glass is positive in most cases, but not when rubbed with 

 mercury in a vacuum, although sealing wax, which is generally negative, 

 is rendered positive by immersion in a trough of mercury. When a white 

 and a black silk stocking are rubbed together, the white stocking acquires 

 positive electricity, and the black negative, perhaps because the black dye 

 renders the silk both rougher and a better conductor. 



Those substances, which have very little conducting power, are some- 

 times called electrics, since they are capable of exhibiting readily the 

 electricity which friction excites on their surfaces, where it remains accu- 

 mulated, so that it may be collected into a conductor ; while the surfaces 

 of such substances, as have greater conducting powers, do not so readily 

 imbibe the fluid from others with which they are rubbed, since they may 

 be supplied from the internal parts of the substances themselves, when 

 their altered capacity requires it; thus, glass, when heated to 110 of 

 Fahrenheit, can with difficulty be excited, becoming an imperfect con- 

 ductor : but a thin plate of a conducting substance, when insulated, may 

 be excited almost as easily as an electric, commonly so called. 



Vapours are generally in a negative state, but if they rise from metallic 

 substances, or even from some kinds of heated glass, the effect is uncertain, 

 probably on account of some chemical actions which interfere with it. 

 Sulfur becomes electrical in cooling, and wax candles are said to be some- 

 times found in a state so electrical, when they are taken out of their 

 moulds, as to attract the particles of dust which are floating near them. 

 The tourmalin, and several other crystallized stones, become electrics^ 



