In a variety of ways, a certain property called electricity can be excited in all bodies, but more readily in some than in others. This property presents itself in two forms, which are called positive and negative electricity, words which we may conveniently use, but which must not be regarded as representing any real knowledge of the distinction between these two kinds of electricity. In fact, let it be remembered throughout, that we do not in the least know what electricity 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 transference is made much more readily to some substances than 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 insulating 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.