The hydraulic safety valve is supposed to have been previously charged with water, and the gas regulator securely attached to the oxygen cylinder.

The blowpipe apparatus is now ready for use, with the taps A, B, and T closed.

First open the cylinder valve by means of the key supplied for that purpose. Then by means of the thumb-screw, P, adjust the pressure of the low-pressure gauge, I, to the correct working pressure for the blowpipe used. Then open the taps A, B, and a, and when acetylene is unmistakably smelt at the nozzle of the blowpipe, ignite it by means of a gas jet, candle, or taper. Then open the tap T, which admits oxygen to the blowpipe, and correct the pressure on the gauge, I (which will be found to have dropped slightly through the tap being opened). Then by means of the tap a slowly throttle down the acetylene until the small white cone of flame at the nozzle of the blowpipe shows a clearly defined outline.

The tap A must never be used to regulate the supply of acetylene; in fact, after the hydraulic safety valve has been charged with water it is best to leave this tap always on.

The pressure of the acetylene supply due to the gas-holder should be not less than 5 ins. of water, and the supply pipe should be proportioned to the maximum quantity required per hour. It is a good plan to fix a water-pressure gauge near the inlet to the hydraulic safety valve in order to note the pressure supply during work.

On stopping work the acetylene taps a or B should be closed first and then the oxygen tap T. When work is completely stopped the oxygen cylinder should be shut off also, and the pressure released from the regulator.

When the apparatus is ready for operation, the welding should be done at the apex or outer extremity of the small bright green cone, which by the Fouche burner retains its radiating brightness and size, about 10 to 15 m.m.

The mixture of gas being ignited at the orifice of the burner, the acetylene will at the moment of combustion with oxygen decompose into its elements, carbon and hydrogen. The carbon takes part in the burning only, while the hydrogen, not being able to combine with the oxygen at the very high temperature in the neighbourhood of the flame, remains temporarily in its free state. The flame consists almost entirely of carbon monoxide, which is being converted at its extremity into carbon dioxide. The free hydrogen forms round the flame a relatively cool jacket, and protects the inner zone from loss of heat, and excludes almost any possibility of oxidation. The small white cone formed in the centre of the flame has at its apex a temperature of about 1,300° Fahr.

In order to obtain a perfect combustion two and a half volumes of oxygen to each volume of acetylene are theoretically required, but in practice it has been found that about equal volumes of the two gases will suffice, dependent, however, upon the purity of the gases and the regulating power of the blowpipe.

The blowpipe should never be kept in such a position as to enable the flame, thrown back from the weld, to strike the head of the burner.

If the flame is not properly regulated, it may fire back and go out. If so, the hydraulic safety valve should be closed at once, and a few seconds allowed to elapse before relighting.

The extinguishing of the flame may be attributed to some of the following reasons: -

(a) Deficiency in the oxygen pressure. See that the oxygen pressure regulator is properly arranged, and that the vessel is open.

(b) Heating of the nozzle of the burner, for instance, by working a deep weld or by placing the blowpipe in a very acute angle, enabling the flame to strike back around the head of the burner. In this case it is advisable to cool the burner in a bucket of water after first turning out the flame.

(c) Choking of the orifice in the nozzle of the burner through beads of iron being splashed into it, or from any other cause, in which case it should be cleaned with a wire brush. No other sharp instrument should be used in the holes.

The great advantage of blowpipe welding is that it is quite as applicable to mild steel as to iron, and that it permits the welding of thin plates, which were heretofore riveted or clinched together.

The blowpipe is further most valuable for use whenever it is necessary to work upon parts already in place, permitting the manufacture of forms of articles requiring numerous and complicated joints, impossible to manufacture by forging. It has the further advantage of being a light apparatus, easily manipulated and requiring no elaborate installation.

At first sight it might appear that the melted or cast metal produced would not have the qualities of strength, and particularly that of elasticity, shown by rolled or hammered material. Surprising results, however, are obtained unless care is taken during the process to avoid overheating or oxidation of the metal, or its alteration by the introduction of impurities such as sulphur or phosphorus. If, for example, thin sheets or tubes of a few millimetres thickness are subjected to a light hammering or even a mild tempering, the metal will be found perfectly ductile, and the weld will exhibit a strength almost equal to the resistance of the original metal. Tubes thus welded may be crushed or twisted, and plates may be bent and refolded, following the weld, without showing any cracks.

When it comes to the welding of comparatively thick plates such as those of boilers, the problem is more complicated, because of the greater difficulty of producing uniform and thorough fusion to a thickness exceeding 6 or 8 m.m. Under such conditions use is often made of an artifice which should be condemned, and which has to some extent discredited autogenous welding of heavy plates.

This artifice consists in chamfering both edges which are to be joined, and filling the space thus left by melting an iron rod in the blowpipe flame. The procedure is not absolutely bad if it is very skilfully carried out and if the operator is careful to use a flame heating a large area and to let the drop of melted metal fall only on the part of the weld which has already been raised to the fusion temperature.