ELECTRTCITY. 373 



740. Light, heat, and electricity appear to have some prop- 

 connection be- erties in common, and each may be made, under certain cir- 



tween light, cumstances, to produce or excite the other. All are so light, 

 heat, and elec- i i- <.,••., 



tricity? Subtle, andditfusive, that it has been found impossible to recog- 



nize in them the ordinary characteristics of matter. Some sup- 

 pose that light, heat, and electricity are all modifications of a common principle. 



741. Electricity exists in, or may be excited in all bodies, 

 electrical di- There are no exceptions to this rule, but electricity is de- 

 ^b°ta* "% ^^ veloped in some bodies with great ease, and in others with 



great difficulty. All substances, therefore, have been divided 

 into two classes, viz.. Electrics, or those which can be easily excited, and 

 Non-electrica, or those which are excited wth difficulty. Such a division is, 

 however, of Uttle practical value in science, and at present is not generally 

 recognized. 



There is no certain test which ^-ill enable us to determine, previous to ex- 

 periment, which of two bodies submitted to friction will produce positive, and 

 which negative electricity. Of all known substances, a cat's fur is the most 

 susceptible of positive, and sulphur of negative electricity. Between these 

 extreme substances others might be so arranged, that any substance in the 

 list being rubbed upon any other, that which holds the highest place will be 

 positively electrified, and that which holds the lower place negatively elec- 

 trified. For instance, smooth glass becomes positively electrified when rub- 

 bed with silk or flannel, but negatively electrified when excited by the back 

 of a living cat. Sealing-wax becomes positive when rubbed with the metals, 

 but negative by any thing else. 



Can one elec- I^ HO CRSG Can electricity of one kind be 

 citeii^wfthout excited without setting free a corresponding 

 othwf ''^^^"'^ amount of electricity of the other kind ; hence, 

 when electricity is excited by friction, the rub- 

 ber always exhibits the one, and the electric, or body 

 rubbed, the other. 



What ara con- 742. Bodics differ greatly in the freedom 

 non!^onductor* ^'^^^ which they allow electricity to pass over 

 of electricity f qj. through them. Those substances which 



regarded as existing by virtue of certain properties or powers, these being merely pecu- 

 liar affections, which may be regarded as being of a similar nature to vibrations. It it 

 assumed that the electric state is but a mode or form of one of these affections. One par- 

 ticle of matter, having received this form of disturbanoe, communicites it to all contigu- 

 ous particles — that is, those which are next to it, although not in contact— and this com- 

 munication of force takes place more or less readily, the communicating particles assuming 

 a polarized state — which may be explained as a stitc presenting two dissimilar extremities. 

 When the communication is slow, the polarized state is bighpst, and the body is said to 

 be an insulator: insulation being the result. If the particles communicate their condition 

 readily, they are termed conductors : conduction is the result. The phenomena of in. 

 duction, or the production of like effects in contiguous bodies, is, therefore, according t* 

 this view, but something analogous to the communication of tremors, or vibratioos." 



