S<2 KLLCTKICITY IN MOTION. 



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 while 

 stocking acquires positive electricity, and the black negative; 

 perhaps because the black dye renders the silk both rougher, and 

 a better conductor. 



Those substanc-es, which have very little conducting power, are 

 sometimes called electrics, since they are capable of exhibiting 

 readily the electricity which friction excites on their surfaces, 

 where it remains accumulated, 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 aro rubbed, since they may be supplied from the 

 internal parts of the substances themselves, when thtir altered 

 capacity requires it; thus, glass, when heated to 110 of Fahren- 

 heit, can with difficulty be excited, becoming an imperfect COD. 

 ductor: but a thin plate of a conducting substance, when in. 

 ^ulated, may be excited almost as easily as au 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. Sulphur becomes electrical in cooling, 

 and wax candles are said to be sometimes found in a state so elec- 

 trical, wheji 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 electrical when heated 

 or cooled, and it is found that the disposition assumed by the 

 fluid, bears a certain relation to the direction in which the stone 

 transmits the light most readily ; some parts of the crystal being 

 rendered always positively and others negatively electrical, by an 

 increase of temperature. 



The most remarkable of the phenomena attending the excitation 

 of electricity of chemical changes, are those which have lately 

 received the appellation of galvanic. Some of the effects which 

 have been considered as belonging to galvanism are probably de- 

 riv. d from the electrical powers of the animal body, and the rest 

 hare been referred by Mr. Volta, and many other philosophers 



