Chapter XI: ), and oxygen coming from another cylinder, are each allowed

to enter the blowpipe at a pressure of 118 to 157 inches of water column (_i.e._, 8.7 to 11.6 inches of mercury; 4.2 to 5.7 lb. per square inch, or 0.3 to 0.4 atmosphere). The gases mix in a chamber tightly packed with porous matter such as that which is employed in the original acetylene reservoir, and finally issue from a jet having a diameter of 1 millimetre at the necessary speed of 100 to 150 metres per second. Finding, however, that the need for having the acetylene under pressure somewhat limited the sphere of usefulness of his apparatus, Fouché finally designed a low-pressure blowpipe, in which only the oxygen requires to be in a state of compression, while the acetylene is drawn directly from any generator of the ordinary pattern that does not yield a gas contaminated with air. The oxygen passes through a reducing valve to lower the pressure under which it stands in the cylinder to that of 1 or 1.5 effective atmosphere, this amount being necessary to inject the acetylene and to give the previously mentioned speed of escape from the blowpipe orifice. The acetylene is led through a system of long narrow tubes to prevent it firing-back.

AUTOGENOUS SOLDERING AND WELDING.--The blowpipe is suitable for the welding and for the autogenous soldering or "burning" of wrought or cast iron, steel, or copper. An apparatus consuming from 600 to 1000 litres of acetylene per hour yields a flame whose inner zone is 10 to 15 millimetres long, and 3 to 4 millimetres in diameter; it is sufficiently powerful to burn iron sheets 8 to 9 millimetres thick. By increasing the supply of acetylene in proportion to that of the oxygen, the tip of the inner zone becomes strongly luminous, and the flame then tends to carburise iron; when the gases are so adjusted that this tip just disappears, the flame is at its best for heating iron and steel. The consumption of acetylene is about 75 litres per hour for each millimetre of thickness in the sheet treated, and the normal consumption of oxygen is 1.7 times as much; a joint 6 metres long can be burnt in 1 millimetre plate per hour, and one of 1.5 metres in 10 millimetre plate. In certain cases it is found economical to raise the metal to dull redness by other means, say with a portable forge of the usual description, or with a blowpipe consuming coal-gas and air. There are other forms of low- pressure blowpipe besides the Fouché, in some of which the oxygen also is supplied at low pressure. Apart from the use of cylinders of dissolved acetylene, which are extremely convenient and practically indispensable when the blowpipe has to be applied in confined spaces (as in repairing propeller shafts on ships _in situ_), acetylene generators are now made by several firms in a convenient transportable form for providing the gas for use in welding or autogenous soldering. It is generally supposed that the metal used as solder in soldering iron or steel by this method must be iron containing only a trifling proportion of carbon (such as Swedish iron), because the carbon of the acetylene carburises the metal, which is heated in the oxy-acetylene flame, and would thereby make ordinary steel too rich in carbon. But the extent to which the metal used is carburised in the flame depends, as has already been indicated, on the proper adjustment of the proportion of oxygen to acetylene. Oxy-acetylene autogenous soldering or welding is applicable to a great variety of work, among which may be mentioned repairs to shafts, locomotive frames, cylinders, and to joints in ships' frames, pipes, boilers, and rails. The use of the process is rapidly extending in engineering works generally. Generators for acetylene soldering or welding must be of ample size to meet the quickly fluctuating demands on them and must be provided with water-seals, and a washer or scrubber and filter capable of arresting all impurities held mechanically in the crude gas, and with a safety vent- pipe terminating in the open at a distance from the work in hand. The generator must be of a type which affords as little after-generation as possible, and should not need recharging while the blowpipe is in use. There should be a main tap on the pipe between the generator and the blowpipe. It does not appear conclusively established that the gas consumed should have been chemically purified, but a purifier of ample size and charged with efficient material is undoubtedly beneficial. The blowpipe must be designed so that it remains sufficiently cool to prevent polymerisation of the acetylene and deposition of the resultant particles of carbon or soot within it.

It is important to remember that if a diluent gas, such as nitrogen, is present, the superior calorific power of acetylene over nearly all gases should avail to keep the temperature of the flame more nearly up to the temperature at which hydrogen and oxygen cease to combine. Hence a blowpipe fed with air and acetylene would give a higher temperature than any ordinary (atmospheric) coal-gas blowpipe, just as, as has been explained in Chapter VI., an ordinary acetylene flame has a higher temperature than a coal-gas flame. It is likely that a blowpipe fed with "Lindé-air" (oxygen diluted with less nitrogen than in the atmosphere) and acetylene would give as high a limelight effect as the oxy-hydrogen or oxy-coal-gas blowpipe.