An instrument, or apparatus, by means of which intelligence is conveyed to any distance with the velocity of lightning. The electric fluid, when an excess has accumulated in one place, always seeks to transfer itself to another, until an equilibrium of its distribution is fully restored. Consequently, when two places are connected by means of a good conductor of electricity, as, for instance, the telegraphic wire; the fluid generated by a galvanic battery, if the communication be rendered complete, instantaneously traverses the whole extent of the wire, and charges, at the distant station, an electro-magnet; this attracts one end of a lever, and draws it downward, while the other extremity is thrown up, and, by means of a style, marks a slip of paper, which is steadily wound off from a roller by the aid of clock-work. If the communication is immediately broken, only one wave of electricity passes over, and a _dot_ is made upon the paper; if kept up, a _line_ is marked. These dots and lines are made to represent the letters of the alphabet, so that an operator employed for the purpose can easily read the message which is transmitted.--The Electro-Magnetic Telegraph was first introduced upon a line between Baltimore and Washington, by Professor Morse, in 1844; at the present time, it is in successful operation between nearly all the important cities and towns of the United States and of Europe. An _Electro-Magnet_ is a piece of soft iron, rendered temporarily magnetic by being placed within a coil of wire through which a current of electricity is passing. CHAPTER XIII. SOAP, CANDLES, TALLOW TREE, SPERMACETI, WAX, MAHOGANY, INDIAN RUBBER OR CAOUTCHOUC, SPONGE, CORAL, LIME, CARBON, OXYGEN, NITROGEN, GAS, HYDROGEN, CHALK, AND MARBLE. Of what is Soap composed? Of soda or potash, and various oily substances; it is so useful for domestic and other purposes, that it may be regarded as one of the necessaries of life; immense quantities of it are consumed in all civilized countries. Soft soap is generally made of a lye of wood-ashes and quicklime, boiled up with tallow or oil; common household soap of soda and tallow, or of potash and tallow; when potash is used, a large portion of common salt, which contains soda, is added to harden it. The finest white soaps are made of olive oil and a lye consisting of soda and quicklime; perfumes are sometimes added, or various coloring matters stirred in to give the soap a variegated appearance. The ancient Greeks and Hebrews appear to have been acquainted with the art of making soap, or a composition very similar to it; and also the ancient Gauls and Germans. A soap-boiler's shop, with soap in it, was found in the city of Pompeii, in Italy, which was overwhelmed by an eruption of Mount Vesuvius, A.D. 79. What is Soda?