EXPERIMENTS WITH SPARK COILS. Experiment 1—An Imitation Gassiot’s Cascade. Wrap a piece of copper wire around a small glass jar and connect it to one of the secondary terminals of the coil. Fill the jar up with water and dip a wire connected to the other secondary terminal into the water. If the coil is then set into operation, the entire outside of the jar will be covered with sparks starting from the wires and rushing upwards into the neck of the jar. If the coil used for this experiment is a very small one, the glass jar should also be very small, as for example, a small medicine vial. This experiment shows off the best when performed in the dark. Experiment 2—A Ghostly Light Grasp the bulb of an old incandescent electric light bulb in one hand and touch the base of the lamp to one of the secondary terminals of the coil. Set the coil in operation and the bulb will glow with a weird greenish light. This experiment will work best in the dark. Experiment 3—Lighting Geissler Tubes. The most beautiful and wonderful effects may be obtained from Geissler tubes operated from a spark coil in a dark room. These tubes come in an almost endless variety of styles and shapes. They are exhausted of air and fitted with metallic wires, sealed into the ends, a portion of the wire projecting outside in the form of small rings, so that the connections are easily established. Some of the tubes are of ordinary glass, while others are made of glass having in its composition certain metallic oxides. Some of the tubes are filled with various gases in a highly rarified state. Others have a double wall with the outside filled with a fluorescent liquid, such as an aqueous solution of sulphate of quinine or an alcoholic solution of strammonium, and other compounds. The tubes are not very expensive. They can be obtained in a great variety of sizes. Most well stocked electrical houses carrying goods for the electrical experimenter have them on hand. Such tubes are of necessity very fragile articles and should be handled with extreme care. It is a good plan to keep them wrapped in cotton and to put them away in a strong wooden box, so as to reduce the danger of breakage. The very smallest coil, even the one-quarter inch size, will serve to light a large tube. . In fact, if too large a coil is used, there is danger of spoiling the tube. The tubes are connected to the spark coil by running a wire from each of the small rings at the ends of the tubes to the secondary terminals. Several tubes may be connected in series when a or 1-inch coil is available. To show the best effects, the room should be in perfect darkness and the tubes should be placed against a background of black velvet. Experiment 4—Flickering Light.