Metals. Lime, coal, or coke. Saline mixtures. Pure water. Vegetable tissues. Animal tissues. Hot air. Steam. Rarefied air. NON-CONDUCTORS. Ice. India-rubber. Marble. Porcelain. Resin. Dry gases. Paper. Wool. Silk. Shell-lac. Diamond. Glass. Wax. Sulphur. [Illustration: Fig. 205.—Positive and negative.] [Illustration: Fig. 206.—The Battery.] It should be observed that the degree of value as a conductor or non-conductor depends somewhat upon the atmosphere. For instance, glass is an excellent insulator, or non-conductor, when dry, but when wet it changes to a conductor. So insulators are at times covered with a solution of shell-lac, or fat, to keep away moisture. We may reasonably conclude that bodies which are good and bad conductors are good and bad conductors of electricity. Water is a good conductor, air is a bad one; were it otherwise, electricity would escape from the ground into the air; as it is, the air manages in some degree to retain the electricity at the surface of bodies, for it is on the surface that we find the electric “fluid.” We have mentioned _electrical induction_ in a former experiment with the tea-tray. We will now explain it more fully, as a consideration of it will bring us to the _electric spark_, or lightning, with the account of the discovery of the _Conductor_ and the _Electrical Machine_. Let us look at the illustration next below. A B is a cylinder supported on a glass rod, and at each extremity is a small pith ball, _a_ and _b_. The cylinder is in a neutral condition, as is evidenced at first by the pellets being in a vertical position. But suppose we bring a ball, C, towards the cylinder. C is charged with positive electricity, which attracts the negative to itself, and so repels the positive away at the opposite side. So the pellet at one side will be attracted to C, and the other will fly in an opposite direction. [Illustration: Fig. 207.—Electrical induction.] Let us take another illustration. Here we have a horizontal metal rod, _cc′_, insulated on a glass stand. Two balls of cork are attached at both ends of the rod by metallic wires. Hold a rod of resin, _r_, which has been made negatively electrical, and apply it to one pair of the cork balls. The positive electricity will be attracted at _c′_ and the negative repulsed, and fly away at _c_. If we remove the resin the equilibrium will be again established, and the balls will fall to a vertical position. [Illustration: Fig. 208.—Induction.] We can also by drawing off the negative electricity by the finger at _c_, while the resin rod is still held to the other side, _c′_, fill the whole of the metal rod with the positive electricity when the finger and the resin have been removed respectively first and last. The balls will then fly in opposite directions again, in consequence of the repulsion exercised by the positive poles.