Spread your fingers out and move your hand rapidly back and forth over a lighted Geissler tube in a darkened room. Your hand will appear to have ten or twelve fingers instead of five. This is due to the fact that the light from a Geissler tube is flickering and not at all steady. Each time the interrupter of the coil opens, the current is sent through the tube and lights it momentarily. The flickering is almost too rapid to be perceived by the naked eye until you have something like the fingers or a pencil in front of it. Experiment 5—Rotating a Geissler Tube. An extremely pretty effect can be secured by rotating a Geissler tube at high speed while it is lighted. This may be accomplished by means of an electric motor of small size. The tube is tied tightly to a stick of wood which has a hole drilled exactly in its center into which the motor shaft fits tightly. Two rings of metal are mounted on the motor shaft, and a wire is led from each to a terminal of the tube. A copper strip rests on each ring, and these two copper strips are connected to the secondary of the spark coil. When the motor is started and the coil set into operation, the tube will appear like the spokes of a wheel. The intermittent light will make the tube visible for a fraction of a second in various positions around the circle as it rotates, and this gives the effect which is much like that of a pinwheel. Experiment 6—Fluorescent Writing. Dissolve some sulphate of quinine in water and then draw a picture or design or write a sentence on a piece of white paper with it, and allow it to dry. The writing will be invisible, but if you hold it up to a lighted Geissler tube in a dark room, it will appear as if written with a beautiful blue ink. Experiment 7—An Electric Bomb. This is a very interesting experiment from which there is no danger if it is performed carefully. The mortar is nothing more or less than a piece of wood which is hard and well seasoned, and has a small hole bored in its axis about three-eighths of an inch in diameter and an inch deep. Two short lengths of stiff copper wire should be inserted through the sides near the bottom of the hole so as to form a small spark gap between the ends. Put a little gunpowder in the mortar and connect two wires from the secondary terminals of the spark coil to the spark gap. Place a cork loosely in the muzzle of the mortar and then, standing well away, close the switch in the primary circuit of the coil, so that a spark passes through the mortar. The spark will ignite the powder and it will explode with a loud report. *Caution*.—Do not use more than a pinch of powder at a time as this is sufficient to cause a sharp explosion. Experiment 8—Electrifying the Garbage Can.