(D) Place E S flat upon the table again; keep it from sliding about with your left hand, and rub it _vigorously_ for a _minute_ with F C. Does E S act exactly as it did before in (B) and (C)? (E) Repeat the experiment in a dark room. (F) Thoroughly electrify E S, and see if it will cling to the wall strongly enough to support its own weight. _=102. Discussion; Electrified and Neutral Bodies.=_ The ebonite sheet became _electrified_ or _charged_; and as the _electrification_ was produced by friction, we may say that the action of the ebonite indicated the presence of _frictional electricity_. No one can tell _just_ why the ebonite acted so queerly, but we can learn a great deal by experimenting. Bodies which are not charged are said to be _neutral_. The table, chairs, earth, etc., are neutral. We may consider that a neutral body has been _discharged_. _=103. Force; Resistance; Work; Potential Energy; Electrification.=_ It takes _force_ to raise water into a tank placed on the roof. In raising the water, _work_ has to be done, and _we_ have to do the work; but when we once have the water in the tank we have accomplished something. The water has _potential energy_; that is, on account of its high _position_, we can make it do some work by simply turning a stop-cock so that the water can run out and turn a water-wheel, for example. It takes _force_ to vigorously rub a piece of ebonite with a flannel cloth, for _resistance_ has to be _overcome_; that is, _work_ has to be done. Several things are accomplished by this work; heat is produced, for we can _feel_ that the ebonite gets warm; we can _hear_ sounds and _see_ sparks. The simple muscular exertion on our part has been changed to heat, light, and sound. The most wonderful part of it all, however, is that we have electrified or charged the ebonite. _We_ did the work at first, and now the ebonite has the power to do something, as you will soon see. _Electrification_ is, then, a sort of potential energy. _=104. Heat and Electrification.=_ We say that heat passes to or from a body to make it hot or cold. Heat _produces_ the sensation of warmth, but heat isn't warmth. We can force a cold body to become hot; in other words, we can get it into a hot condition in various ways, such as rubbing it, hammering it, or by placing it near or in contact with another hot body. Electrification is, also, a condition or state into which we can force a body; but electrification isn't electricity. We know whether a body is hot or cold by its effects upon us, upon thermometers, and upon other bodies. We can tell, also, whether a body is electrified or not by the way it acts, and, in certain cases, by the sound, heat, and light which accompany the electrification. Do not get the idea that an electrified body is covered with a layer of electricity just as a board is covered with a layer of paint. [Illustration: Fig. 28.] =EXPERIMENT 52.=