Transparent precious stones. Diamond. Mica. All vitrefactions. Glass. Jet. Wax. Sulphur. Resins. Amber. Gutta-percha. Shell-lac (or gum-lac). Ebonite. There are, as we know, two kinds of electricity, the _static_ and _dynamic_; and when the latter state is _instantaneous_, it is referred to as the “electric discharge,” which occurs when opposite electricities seek each other, and the bodies return to a state of equilibrium or neutralization. “These bodies, if _insulated_, obtain no more electricity after the spark has passed; but if there be a constant source of negative electricity supplying one, and a constant source of negative electricity supplying the other, there will be a succession of sparks; and if they communicate by a conductor there will be, through this conductor, an uninterrupted neutralization of a continual reunion of the two electricities, and this is what is termed the continuous dynamic state or electric current” (De la Rive). FOOTNOTES: [13] This interesting experiment, which we have exactly verified, was described to us by Professor Waldner, and M. A. Keppler. CHAPTER XIX. VELOCITY OF ELECTRICITY—EXPERIMENTS—THE ELECTRIC EGG—FORCE OF THE ELECTRIC SPARK. We are now acquainted with many facts concerning electricity, and have seen that electrical phenomena can be produced by the Electric Machine and the Leyden Jar. (An insulating stool—a stool with glass legs—is a very desirable adjunct for those who wish to experiment with the machine). Glass is a great insulator, or non-conductor, as a Russian philosopher found to his cost. He had an iron lightning-conductor from his house into his room, the end not connected with the earth but with a glass. One day the lightning came down the rod and reached the glass; had a communication been made with the earth by a chain, or directly, no mischief would have ensued. As it happened, however, the current was checked by the glass, and immediately darted towards him; it struck him in the head, and killed the poor man on the spot. If no insulating stool were used, the body charged would be discharged upon contact with the ground.