_Materials._--For the performance of the above operations we require
the two gases, oxygen and acetylene, to produce the flames; rods of metal
which may be added to the joints while molten in order to give the weld
sufficient strength and proper form, and various chemical powders, called
fluxes, which assist in the flow of metal and in doing away with many of
the impurities and other objectionable features.

_Instruments._--To control the combustion of the gases and add to the
convenience of the operator a number of accessories are required.

The pressure of the gases in their usual containers is much too high for
their proper use in the torch and we therefore need suitable valves which
allow the gas to escape from the containers when wanted, and other
specially designed valves which reduce the pressure. Hose, composed of
rubber and fabric, together with suitable connections, is used to carry the
gas to the torch.

The torches for welding and cutting form a class of highly developed
instruments of the greatest accuracy in manufacture, and must be thoroughly
understood by the welder. Tables, stands and special supports are provided
for holding the work while being welded, and in order to handle the various
metals and allow for their peculiarities while heated use is made of ovens
and torches for preheating. The operator requires the protection of
goggles, masks, gloves and appliances which prevent undue radiation of the
heat.

_Torch Practice._--The actual work of welding and cutting requires
preliminary preparation in the form of heat treatment for the metals,
including preheating, annealing and tempering. The surfaces to be joined