end submerged in the molten metal of the weld at all times.

When aluminum is melted it forms alumina, an oxide of the metal. This
alumina surrounds small masses of the metal, and as it does not melt at
temperatures below 5000 degrees (while aluminum melts at about 1200), it
prevents a weld from being made. The formation of this oxide is retarded
and the oxide itself is dissolved by a suitable flux, which usually
contains phosphorus to break down the alumina.

_Copper._--The whole piece should be preheated and kept well covered
while welding. The flame must be much larger than for the same thickness of
steel and neutral in character. A slight excess of acetylene would be
preferable to an excess of oxygen, and in all cases the molten metal should
be kept enveloped with the flame. The welding rod is of copper which
contains phosphorus; and a flux, also containing phosphorus, should be
spread for about an inch each side of the joint. These assist in preventing
oxidation, which is sure to occur with heated copper.

Copper breaks very easily at a heat slightly under the welding temperature
and after cooling it is simply cast copper in all cases.

_Brass and Bronze._--It is necessary to preheat these metals, although
not to a very high temperature. They must be kept well covered at all times
to prevent undue radiation. The flame should be produced with a nozzle one
size larger than for the same thickness of steel and the small blue-white
cone should be held from one-fourth to one-half inch above the surface of
the work. The flame should be neutral in character.

A rod or wire of soft brass containing a large percentage of zinc is
suitable for adding to brass, while copper requires the use of copper or