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.
If you are ever annoyed by dogs or cats that knock the cover off the garbage can in their search for canine and feline delicacies, you can give them the surprise of their lives with the aid of a spark coil upon the occasion of their next visit.
Set the can upon a thick piece of perfectly dry wood and run a wire from the can to one secondary terminal of the spark coil. Connect the other secondary terminal to the ground or to a water or gas pipe.