nt of the bit and to carry the
tinning back at least half an inch from the edge.

If the solder obstinately refuses to adhere, the cause is to be sought
in the oxidation of the copper, or of the solder, or both--in either
case the result of too high a temperature or too prolonged heating.
The simple remedy is to get the iron hot, and then to dress it with an
old file, so as to expose a bright surface, which is instantly passed
over the resin as a means of preserving it from oxidation. If the
process above described be now carried out, it will be found that the
difficulty disappears.

Before using the iron, wipe off any soot or coke or burned resin by
means of an old rag. An iron tinned in this way is much to be
preferred to one tinned by means of chloride of zinc.

A shorter and more usual method is carried out as follows: The
solution of chloride of zinc is prepared by adding bits of zinc to
some commercial hydrochloric acid diluted with a little (say 25 per
cent) of water. The acid may conveniently be placed in a small glazed
white jar (a jam pot does excellently), and this should only be filled
to about one-quarter of its capacity. An excess of zinc may be added.

It may be fancy, but I prefer a soldering solution made in this way to
a solution of chloride of zinc bought as a chemical product. The jar
is generally mounted on a heavy leaden base, so as to avoid any danger
of its getting knocked over, for nothing is so nasty or bad for tools
as a bench on which this noxious liquid has been upset (Fig. 78).

Fig. 78.

To tin a soldering bit, a little of the fluid is dipped out of the jar
on to a bit of tin plate bent up at the edges--a few drops is
sufficient--and the iron is heated and rubbed about in the liquid
with a drop of solder. If the iron is anything like clean it will tin
at once and exhibit a very bright surface, but quite dirty copper may
be tinned by dipping it for a moment in the liquid in the pot and then
working it about over the solder. An iron so tinned remains covered
with chloride of zinc, and this must be carefully wiped off if it is
intended to use the iron with a resin or tallow flux in lead
soldering.

One disadvantage of this process is that the copper bit soon gets
eaten into holes and requires to be dressed up afresh. On the other
hand, an iron so tinned always presents a nice clean solder surface
until the next time it is heated, when it generally becomes very dirty
and req