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Hydrogen finds numerous uses in modern industry. In recent years its importance has greatly increased, owing to its use in the hardening of oils. An air-hydrogen blowpipe is used for the autogenous welding of lead sheets in the making of vitriol chambers; pure lead is used as a solder, being melted over the junction by the flame ("lead burning"). The oxy-hydrogen (or oxy-coal gas) blowpipe is used in fusing quartz in making fused silica apparatus, e.g., mercury lamps, or for fusing platinum. A mixture of hydrogen and nitrogen is used in the Haber process for the synthetic production of ammonia: N2 + 3H2 <=> 2NH3. Hydrogen is used in filling balloons and the gas containers of airships, i cu. m. of air weighs 1.29 kgm., 1 cu. m. of hydrogen weighs 0.09 kgm., so that each cu. m. of space filled with hydrogen exerts in air a lifting force of 1.29 – 0.09 = 1.2 kgm. The danger of fire in such cases is great, and in America hydrogen is replaced by helium. The hydrogen used in military balloons is usually transported in cylinders.
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