=223. The Electron Theory.=--Various discoveries and experiments made in recent years indicate, however, that _negative_ electricity consists of little _corpuscles_ or _electrons_ which may pass readily from one molecule of a conductor to another while their movement through an insulator is much retarded if not entirely prevented. This theory, sometimes called the _Electron Theory_, holds that each atom of a substance has as a nucleus a corpuscle of _positive_ electricity, and surrounding it, minute negative corpuscles or electrons. It is thought that the electrons in the atom are very much smaller than the positive charges and are revolving about the latter with great rapidity. Ordinarily, the positive and negative charges are equal so that the atom is in a neutral or uncharged condition. By the action of various forces some of the _negative_ corpuscles within a conductor may be moved from molecule to molecule. Thus if a negatively charged rod is brought near a conductor, many electrons stream away to the far end charging it _negatively_, while the nearer end of the conductor is left with fewer electrons than usual along with the fixed positive corpuscles. Hence the near end is positively charged. (See Fig. 198.) On the other hand, if a positive charge is used, it attracts the electrons from the far end, leaving the immovable positive corpuscles there, and that end becomes positively electrified, while the nearer end with its surplus of electrons is, of course, negatively electrified. The Electron Theory is considered well founded since the electrons have (a) had their _mass_ determined, (b) their _speed_ measured, (c) their _electric charge_ determined, (d) and their _behavior_ while _passing through magnetic_ and _electric fields_ observed. These facts and other experimental evidence have demonstrated the existence of electrons. The positive corpuscle has not been directly observed but is assumed to exist to account for the effects observed in induction, charging by friction, etc.