The jar must be thoroughly cleaned and dried before coating. The inside is then given a thorough brushing over with shellac or varnish. Before it is dry, carefully insert the tin-foil and press it smoothly against the glass. The outside of the jar is treated and coated in the same manner. The inside and outside of the bottom are also coated by cutting the tinfoil in circular pieces and shellacking them on. In order to charge the Leyden jar, grasp it in the hand near the bottom and hold the knob against the prime conductor while turning the handle of the machine. [Illustration: Fig. 46.—A Wooden Mortar for Igniting Gunpowder.] *Igniting gunpowder.* Bore a hole one-half inch in diameter and one inch deep in a block of hardwood. Pass two small brass wires through holes in the sides, letting the ends of the wires be about one-eighth of an inch apart. Pour a little gunpowder in loosely over the wires. Tie a piece of thoroughly moistened cotton twine, three inches long, to one of the wires and attach it to the outside coating of a charged Leyden jar. Connect the knob of the jar to the other wire. The gunpowder will immediately explode. Keep the face and hands away from the gunpowder when performing this experiment. [Illustration: Fig. 47.—An Electric Umbrella.] *Electric Umbrella.* The repulsion of similarly electrified bodies which was illustrated by the action of the pith ball electroscope may be better illustrated by pasting some narrow streamers of tissue paper about one-eighth of an inch wide and four inches long to a small cork covered with tinfoil. The cork is mounted on the upper end of a stiff copper wire supported in a bottle. When the wire is connected to the prime conductor and the machine set in motion, the strips will spread out like an umbrella. *Lightning Board.* A pane of glass is thoroughly cleaned and then given a coat of shellac or varnish. Before the varnish is dry, press on a piece of tinfoil large enough to cover one side of the glass and rub it down smoothly. [Illustration: Fig. 48.—A Lightning Board.] After the shellac or varnish is dry, cut the tinfoil up into innumerable little squares with a sharp knife and ruler, leaving two solid strips of tinfoil at the ends of the glass pane. The pane is mounted by cementing it in a slot in the cork of a bottle. Connect one of the tinfoil strips to the prime conductor and the other to the earth or the body. When the machine is turned, innumerable little sparks will pass between the tinfoil squares and give an appearance very similar to that of lightning. [Illustration: Fig. 49.—An Electric Dance.]