occasionally be replaced by another in which the moulding material
is a mixture of treacle and glue. The glue is soaked in cold water
till it is completely soft. The superfluous water thrown away,
one-fourth part by volume of thick treacle is added, and the mixture
is melted on the water bath; during which process stirring has to be
resorted to, to produce a uniform mixture.

This liquid forms the moulding mixture, and it is allowed to flow
round the object to be copied, contained in a suitable box, whose
sides have been slightly oiled. The object to be copied should also
be oiled. After some hours, when the glue mixture has set, it will be
found to be highly elastic, so that it may be pulled away from the
mould, and afterwards resume very nearly its original form.

One drawback to the use of these moulds lies in the fact that the
gelatine will rarely stand the plating solution without undergoing
change, but this may be partially obviated by dipping it for a few
seconds in a 10 per cent solution of bichromate of potash, exposing it
to the sunlight for a few minutes, and then rinsing it.

In order to render the surface conducting, it is washed over with a
solution of a gold or silver salt, and the latter reduced in situ to
metal by a suitable reagent. A solution of phosphorus is the most
usual one (see Gore, Electro-metallurgy, p. 216). Such a mould may
be copper-plated in the sulphate bath, connection being made by wires
suitably thrust into the material.

Plaster of Paris moulds require to be dried and waxed by standing on a
hot plate in melted wax before they are immersed in the plating bath.
In this case the surface is best made conducting either by silvering
it by the silvering process used for mirrors, or by brushing it over
with good black lead rendered more conducting by moistening with an
ethereal solution of chloride of gold and then drying in the sun.

The brushing requires a stiff camel's-hair pencil of large size cut so
that the hairs project to a distance of about a quarter of an inch
from the holder. The brushing must continue till the surface is
bright, and is often a lengthy process.

The same process of blackleading may be employed to get a coat of
deposited metal which will strip easily from the cathode.

In all cases where extensive deposits of copper are required, the
growth takes place too rapidly at the corners. Consequently it is
often desirable to localise the action of the depos