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Langmuir (1912) has shown that hydrogen in contact with a tungsten wire heated by an electric current at low pressure, is dissociated into atoms: H2 <=> 2H. This splitting of the hydrogen molecule is attended by the absorption of a large amount of energy, about 100 k. cal. per gram-molecule. The atomic hydrogen so formed is chemically very active. Langmuir also showed that atomic hydrogen is formed when an electric arc between tungsten electrodes is allowed to burn in hydrogen at atmospheric pressure
 | Fig: The atomic hydrogen blowpipe Hydrogen gas issues from the central nozzle on the right and is dissociated in an electric arc between two tungsten rods nearly meeting in a V. |
The atomic hydrogen was blown out of the arc by a jet of molecular hydrogen directed across the arc, and formed an intensely hot flame, which is capable of melting tungsten (m. pt. 3400°). This flame obtains its heat not from combustion but from the recombination of hydrogen atoms to H2. It is suitable for melting and welding many metals. Iron can be melted without contamination with carbon, oxygen, or nitrogen. Because of the powerful reducing action of the atomic hydrogen, alloys can be melted without fluxes and without surface oxidation. A feature of the flame is the great rapidity with which heat can be delivered to a surface, which is very important in welding operations.
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