f parts.

UNITING METALS.--Metals are united in three ways, where heat is used:

First: By heating two or more of them to such a high temperature that
they melt and form a compound, or an alloy, as it is called.

Second: By heating up the points to be joined, and then lapping the
pieces and hammering the parts. This is called forge work or welding.

Third: By not heating the adjacent parts and using an easily fusible
metal, which is heated up and run between the two, by means of a
soldering iron.

The foreign material used in the first is called a flux; in the second
it is termed a welding compound; and in the third it is known as a
soldering acid, or soldering fluid.

The boy is not so much interested in the first process, from the
standpoint of actual work, but it is necessary that he should have some
understanding of it.

It may be said, as to fluxes, generally, that they are intended to
promote the fusion of the liquefying metals, and the elements used are
the alkalis, such as borax, tartar, limestone, or fluor spar.

These substances act as reducing or oxidizing agents. The most important
are carbonate of soda, potash, and cyanide of potassium. Limestone is
used as the flux in iron-smelting.

WELDING COMPOUNDS.--Elsewhere formulas are given of the compounds most
desirable to use. It is obvious that the application of these substances
on the heated surfaces, is not only to facilitate the heating, but to
prepare the articles in such a manner that they will more readily adhere
to each other.

OXIDATION.--Oxidation is the thing to guard against in welding. The
moment a piece of metal, heated to whiteness, is exposed, the air coats
it with a film which is called an _oxide_. To remove this the welding
compound is applied.

The next office of the substance thus applied, is to serve as a medium
for keeping the welding parts in a liquid condition as long as possible,
and thus facilitate the unity of the joined elements.

When the hammer beats the heated metals an additional increment of heat
is imparted to the weld, due to the forcing together of the molecules of
the iron, so that these two agencies, namely, the compound and the
mechanical friction, act together to unite the particles of the metal.

SOLDERING.--Here another principle is involved, namely, the use of an
intermediate material between two parts which are to be united. The
surfaces to be brought together must be thoroughly cleaned, using such
age