Nickel-plating is another extensive branch of the industry, the white nickel forming a cloak for metals more subject to corrosion.  Nickel is found to deposit best from a solution of the double sulphate of nickel and ammonia.  Aluminium, however, has not yet been successfully deposited by electricity.

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.