In 1836 De la Rue observed that copper laid in this manner on another surface took on its under side an accurate impression of that surface, even to the scratches on it, and three years later Jacobi, of St. Petersburg, and Jordan, of London, applied the method to making copies or replicas of medals and woodcuts. Even non-metallic surfaces could be reproduced in copper by taking a cast of them in wax and lining the mould with fine plumbago, which, being a conductor, served as a cathode to receive the layer of metal. It is by the process of electrotyping or galvano- plastics that the copper faces for printing woodcuts are prepared, and copies made of seals or medals.
Natural objects, such as flowers, ferns, leaves, feathers, insects, and lizards, can be prettily coated with bronze or copper, not to speak of gold and silver, by a similar process. They are too delicate to be coated with black lead in order to receive the skin of metal, but they can be dipped in solutions, leaving a film which can be reduced to gold or silver. For instance, they may be soaked in an alcoholic solution of nitrate of silver, made by shaking 2 parts of the crystals in 100 parts of alcohol in a stoppered bottle. When dry, the object should be suspended under a glass shade and exposed to a stream of sulphuretted hydrogen gas; or it may be immersed in a solution of 1 part of phosphorus in 15 parts of bisulphide of carbon, 1 part of bees-wax, 1 part of spirits of turpentine, 1 part of asphaltum, and 1/8 part of caoutchouc dissolved in bisulphide of carbon. This leaves a superficial film which is metallised by dipping in a solution of 20 grains of nitrate of silver to a pint of water. On this metallic film a thicker layer of gold and silver in different shades can be deposited by the current, and the silver surface may also be "oxidised" by washing it in a weak solution of platinum chloride.
Electrolysis is also used to some extent in reducing metals from their ores, in bleaching fibre, in manufacturing hydrogen and oxygen from water, and in the chemical treatment of sewage.
CHAPTER VI.
THE TELEGRAPH AND TELEPHONE.
Like the "philosopher's stone," the "elixir of youth," and "perpetual motion," the telegraph was long a dream of the imagination. In the sixteenth century, if not before, it was believed that two magnetic needles could be made sympathetic, so that when one was moved the other would likewise move, however far apart they were, and thus enable two distant friends to communicate their minds to one another.
The idea was prophetic, although the means of giving effect to it were mistaken. It became practicable, however, when Oersted discovered that a magnetic needle could be swung to one side or the other by an electric current passing near it.
The illustrious Laplace was the first to suggest a telegraph on this principle. A wire connecting the two poles of a battery is traversed, as we know, by an electric current, which makes the round of the circuit, and only flows when that circuit is complete. However long the wire may be, however far it may run between the poles, the current will follow all its windings, and finish its course from pole to pole of the battery. You may lead the wire across the ocean and back, or round the world if you will, and the current will travel through it.
The moment you break the wire or circuit, however, the current will stop. By its electromotive force it can overcome the resistance of the many miles of conductor; but unless it be unusually strong it cannot leap across even a minute gap of air, which is one of the best insulators.
If, then, we have a simple device easily manipulated by which we can interrupt the circuit of the battery, in accordance with a given code, we shall be able to send a series of currents through the wire and make sensible signals wherever we choose. These signs can be produced by the deviation of a magnetic needle, as Laplace pointed out, or by causing an electro-magnet to attract soft iron, or by chemical decomposition, or any other sensible effect of the current.