a wire and dipped into
a solution of a silver salt. Electrical connection is made in
such a way that the article to be plated serves as the cathode,
while the anode is made up of one or more plates of silver
(Fig. 88, A). When a current is passed through the
electrolyte silver dissolves from the anode plate and deposits
on the cathode in the form of a closely adhering layer. By
making the proper change in the electrolyte and anode plate
objects may be plated with gold and other metals.

~Compounds of silver.~ Silver forms two oxides but only one series of
salts, namely, the one which corresponds to the mercurous and cuprous
series.

~Silver nitrate~ (_lunar caustic_) (AgNO_{3}). This salt is easily
prepared by dissolving silver in nitric acid and evaporating the
resulting solution. It crystallizes in flat plates, and when heated
carefully can be melted without decomposition. When cast into sticks it
is called lunar caustic, for it has a very corrosive action on flesh,
and is sometimes used in surgery to burn away abnormal growths.

The alchemists designated the metals by the names of the
heavenly bodies. The moon (luna) was the symbol for silver;
hence the name "lunar caustic."

~Silver sulphide~ (Ag_{2}S). This occurs in nature and constitutes one of
the principal ores of silver. It can be obtained in the form of a black
solid by passing hydrosulphuric acid through a solution of silver
nitrate.

~Compounds of silver with the halogens.~ The chloride, bromide, and iodide
of silver are insoluble in water and acids, and are therefore
precipitated by bringing together a soluble halogen salt with silver
nitrate:

AgNO_{3} + KCl = AgCl + KNO_{3}.

They are remarkable for the fact that they are very sensitive to the
action of light, undergoing a change of color and chemical composition
when exposed to sunlight, especially if in contact with organic matter
such as gelatin.

~Photography.~ The art of photography is based on the fact that
the halogen compounds of silver are affected by the light,
particularly in the presence of organic matter. From a chemical
standpoint the processes involved may be described under two
heads: (1) the preparation of the negative; (2) the preparation
of the print.

1. _Preparation of the negative._ The plate used in the
preparation of the negative is made by spreading a thin layer
o