Through the effects of a phenomenon called _induction,_ and which we have already stumbled against in the experiment with the tacks and the magnetic chain, the force exerted by the charge grows stronger because of a charge of the opposite kind on a neighboring cloud or some object on the earth beneath. These charges continually strive to burst across the intervening air. As soon as the charge grows strong enough a vivid flash of lightning, which may be from one to ten miles long, takes place. The heated air in the path of the lightning expands with great force; but immediately other air rushes in to fill the partial vacuum, thus producing the terrifying sounds called _thunder_. In the eighteenth century, electricity was believed to be a sort of fiery atmospheric discharge, as has been said. Later it was discovered that it seemed to flow like water through certain mediums, and so was thought to be a fluid. Modern scientists believe it to be simply a vibratory motion, either between adjacent particles or in the ether surrounding those particles. It was early discovered that electricity would travel through some mediums but not through others. These were termed respectively "conductors" and "non-conductors" or insulators. Metals such as silver, copper, gold, and other substances like charcoal, water, etc., are good conductors. Glass, silk, wool, oils, wax, etc., are non-conductors or insulators, while many other substances, like wood, marble, paper, cotton, etc., are partial conductors. There seems to be two kinds of electricity, one called "static" and the other "current" electricity. The former is usually produced by friction while the latter is generated by batteries or dynamos. A very simple and well-known method of generating static electricity is by shuffling or sliding the feet over the carpet. The body will then become _charged_, and if the knuckles are presented to some metallic object, such as a gas-jet or radiator, a stinging little spark will jump out to meet it. [Illustration: _From the author’s "Wireless Telegraphy and Telephony" by permission._ *A Double Lightning Discharge from a Cloud to the Earth.*] The electricity is produced by the friction of the feet sliding over the carpet and causes the body to become electrified. Warm a piece of writing-paper, then lay it on a wooden table and rub it briskly with the hand. It soon will become stuck to the table and will not slide along as it did at first. If one corner is raised slightly it will tend to jump right back. If the paper is lifted off the table it will tend to cling to the hands and the clothing. If held near the face it will produce a tickling sensation. All these things happen because the paper is electrified. It is drawn to the other objects because they are _neutral_, that is, do not possess an electrical charge. [Illustration: Fig. 19.—A Piece of Dry Writing-Paper may be Electrified by Rubbing.]