the least speck of rust is fatal to a
burnisher. In any case the steel requires to be occasionally
repolished by rouge and water on a bit of cloth or felt. The process
of burnishing is necessarily somewhat slow and tedious, and as a rule
is not worth troubling about except in cases where great permanence is
required.

The burnisher is moved over the work somewhat like a pencil with
considerable pressure, and care is taken to make the strokes as
uniform in direction as possible; otherwise the surface looks
non-uniform, and has to be further polished by tripoli, whitening,
etc, before it is presentable.

Sec. 132. Silver-plating.

The most convenient solution for general purposes is an 8 to 10 per
cent solution of the double cyanide of silver and potassium together
with 1 or 2 per cent of "free" potassium cyanide. Great latitude is
permissible in the strength of solution and density of current. As
commercial cyanide of potassium generally contains an unknown
percentage of other salts, which, however, do not interfere with its
value for the purpose of silver-plating, the simplest procedure is as
follows.

For every 100 c.c. of plating solution about 7 grms. of dry
crystallised silver nitrate are required. The equivalent amount of
potassium cyanide (if dry and pure) is 5.2 grms, but commercial
cyanide may contain from 50 per cent upwards to 96 per cent in the
best fused cyanide made from ferrocyanide only. An approximate idea
of the cyanide content can be obtained from the dealers when the salt
is purchased, and this is all that is required.

A quantity slightly in excess of the computed amount of cyanide is
dissolved in distilled water, and this is cautiously added to the
solution of the silver nitrate till precipitation is just complete.
The supernatant liquors are then drained away, and the precipitate
dissolved by adding a sufficiency of the remaining cyanide; this
process is assisted by warming and stirring.

An allowance of about one-tenth of the whole cyanide employed may be
added to form "free" cyanide, and the solution made up to the strength
named. It is advisable to begin with the cyanide in a moderately
strong solution, for the sake of ease in dissolving the precipitate.

This solution will deposit silver upon articles of copper or brass
immersed in it even without the battery, but the coat will be thin.
The solution is used cold, with a current density of about 10 to 20
amperes per square f