in the small practice which
occurs in the laboratory a solution prepared as suggested does
perfectly for everything except iron or steel. The scratch-brushing
should be done over a large photographic developing dish to avoid loss
of gold. It is a good plan to rinse the articles after leaving the
bath in a limited quantity of distilled water, which is afterwards
placed in a "residue" bottle, and then to scratch-brush them by hand
over the dish to catch fine gold. When any loose dust is removed the
articles may be scratched in the lathe without appreciable further
loss.

Silver-gilt articles tend to get discoloured by use, but this
discoloration can be removed by soap and water. After long use a gold
cyanide bath tends to alter greatly in composition, In general, the
bath tends to grow weaker, from the fact that there is a strong
temptation to gild as many articles at once as possible.

It is therefore a good plan to keep a rough profit and loss account of
the gold in order to find the quantity in solution. Fifty dwts. per
gallon (or 78 grms. per 4.5 litres) is recommended. A gallon of
solution of this strength is worth about eleven pounds sterling in
gold and cyanide, and a serviceable anode will be worth about 10
pounds. (Fine gold is worth nominally four pounds four shillings and
eleven pence ha'penny per oz.) Gold may be easily obtained containing
less impurity than one part in ten thousand.

Sec. 137. Plating with Copper.

Copper may be deposited from almost any of its salts in reguline form,
the sulphate and nitrate being most usually employed. In the
laboratory a nearly saturated solution of sulphate of copper with 1 or
2 per cent of sulphuric acid will answer most purposes. A current
density of, at most, fifteen amperes per square foot may be used,
either for obtaining solid deposits for constructional purposes or for
calibrating current measuring instruments by electrolysis. A copper
anode is of course employed.

When coppering with a view to obtaining thick deposits it is a good
plan to place the electrodes several inches apart, and, if possible,
to keep the liquid stirred, as there is a considerable tendency on the
part of copper deposits to grow out into mossy masses wherever the
current density exceeds the limit mentioned. As the masses grow
towards the anode the defect naturally tends to increase of itself,
hence the necessity for care. The phenomenon is particularly marked
at the edges