The experiments of figures 1, 2, and 3 have also shown us that when the pithball is charged with the positive electricity of the glass rod it is repelled by the like charge upon the rod, and attracted by the negative or unlike charge on the handkerchief.  Again, when it is charged with the negative electricity of the handkerchief it is repelled by the like charge on the handkerchief and attracted by the positive or unlike charge on the rod.  Therefore it is usual to say that like electricities repel and unlike electricities attract each other.

We have said that all bodies yield electricity under the friction of dissimilar bodies; but this cannot be proved for every body by simply holding it in one hand and rubbing it with the excitor, as may be done in the case of glass.  For instance, if we take a brass rod in the hand and apply the rubber vigorously, it will fail to attract the pithball, for there is no trace of electricity upon it.  This is because the metal differs from the glass in another electrical property, and they must therefore be differently treated.  Brass, in fact, is a conductor of electricity and glass is not.  In other words, electricity is conducted or led away by brass, so that, as soon as it is generated by the friction, it flows through the hand and body of the experimenter, which are also conductors, and is lost in the ground.  Glass on the other hand, is an insulator, and the electricity remains on the surface of it.  If, however, we attach a glass handle to the rod and hold it by that whilst rubbing it, the electricity cannot then escape to the earth, and the brass rod will attract the pith-ball.

All bodies are conductors of electricity in some degree, but they vary so enormously in this respect that it has been found convenient to divide them into two extreme classes—­conductors and insulators.  These run into each other through an intermediate group, which are neither good conductors nor good insulators.  The following are the chief examples of these classes:—­

Conductors.—­All the metals, carbon.

Intermediate (bad conductors and bad insulators).—­Water, aqueous solutions, moist bodies; wood, cotton, hemp, and paper in any but a dry atmosphere; liquid acids, rarefied gases.

Insulators.—­Paraffin (solid or liquid), ozokerit, turpentine, silk, resin, sealing-wax or shellac, india-rubber, gutta-percha, ebonite, ivory, dry wood, dry glass or porcelain, mica, ice, air at ordinary pressures.