uires to be carefully wiped before using.

In my experience also an iron so tinned is more easily spoiled as to
the state of its surface, "detinned," in fact, by overheating than
when the tinning is carried out by resin and friction. When this
happens, the shortest way out of the difficulty is the application of
the old file so as to obtain a perfectly fresh surface. No one who
knows his business ever uses an iron that is not perfectly clean and
well tinned.

The iron may be cleaned from time to time by heating it red hot and
quenching it in water to get rid of the oxide, which scales off in the
process.

Sec. 95. Soft Soldering.

In the laboratory the chief application of the process is to copper
soldering during the construction of electrical apparatus and to zinc
soldering for general purposes.

In ninety-nine cases out of every hundred where difficulties occur
their origin is to be traced to dirt. There seems to be some
inexplicable kink in the human mind which renders it callous to
repeated proofs of the necessity for cleaning surfaces which it is
intended to solder. The slightest trace of albuminous or gelatinous
matter or shellac will prevent solder adhering to most metals and the
same remark applies in a measure to the presence of oxides, although
these may be removed by chloride of zinc or prevented from forming by
resin or tallow. A touch with an ordinarily dirty hand--I refer to a
solderer's hand--will often soil work sufficiently to make the
adherence of solder difficult.

The fluxes most generally employed are tallow for lead, resin or
Venice turpentine for copper, chloride of zinc for anything except
lead, which never requires it. The latter flux has the property (also
possessed by borax at a red heat) of dissolving any traces of oxide
which may be formed, as well as acting as a protecting layer to the
metal.

We may now turn to the consideration of a simple case of soldering,
say the joining of two copper wires. The wires are first cleaned
either by dipping in a bath of sulphuric and nitric acids--a thing no
laboratory should be without--or by any suitable mechanical means.
The cleaned wires are then twisted together--there is a regulation
way of doing this, but it presents no advantage in laboratory
practice--and the joint is sprinkled over with resin, or painted
with a solution of resin in alcohol.

The iron, being heated and floated with solder, is held against the
joint, the latter