238 PLEASANT WA YS IN SCIENCE. 



tricity is ; we only know certain of the phenomena which 

 it produces. Any body which has become charged with 

 electricity, either positive or negative, will part with its 

 charge to bodies in a neutral condition, or charged with the 

 opposite electricity (negative or positive). But the trans- 

 ference is made much more readily to some substances th^n 

 to others so slowly, indeed, to some, that in ordinary 

 experiments the transference may be regarded as not taking 

 place at all. Substances of the former kind are called good 

 conductors of electricity ; those which receive the transfer 

 of electricity less readily are said to be bad conductors ; 

 and those which scarcely receive it at all are called insu- 

 lating substances. The reader must not confound the 

 quality I am here speaking of with readiness to become 

 charged with electricity. On the contrary, the bodies which 

 most freely receive and transmit electricity are least readily 

 charged with electricity, while insulating substances are 

 readily electrified. Glass is an insulator, but if glass is 

 briskly rubbed with silk it becomes charged (or rather, the 

 part rubbed becomes charged) with positive electricity, 

 formerly called vitreous electricity for this reason ; and 

 again, if wax or resin, which are both good insulators, be 

 rubbed with cloth or flannel, the part rubbed becomes 

 charged with negative, formerly called resinous, electricity. 



Electricity, then, positive or negative, however generated, 

 passes freely along conducting substances, but is stopped 

 by an insulating body, just as light passes through trans- 

 parent substances, but is stopped by an opaque body. 

 Moreover, electricity may be made to pass to any distance 

 along conducting bodies suitably insulated. Thus, it might 

 seem that we have here the problem of distant communica- 

 tion solved. In fact, the first suggestion of the use of elec- 

 tricity in telegraphy was based on this property. When a 

 charge of electricity has been obtained by the use of an 

 ordinary electrical machine, this charge can be drawn off at 

 a distant point, if a conducting channel properly insulated 

 connects that point with the bodies (of whatever nature) 



