Cavendish, who called hydrogen "inflammable air", was also the first to measure its density. He reported in his 1766 paper that hydrogen is 7 to 11 times lighter than air (the correct value is 14.4). Cavendish's results not only opened up a new chapter in the history of gases, but also attracted attention to hydrogen as an alternative to hot air as a buoyant gas. Jacques Alexandre Cesar Charles was the first to take advantage of this, soon after the first public demonstration by the Mongolfier brothers on 15 th June 1783 of a hot-air balloon . After four days of struggling with his iron-acid hydrogen generation equipment, Charles launched his 4-m balloon on 27th August 1783. Just over three months later, he and one of his balloon builders, Aine Robert, became the first men to ascend in a hydrogen balloon. With all the enthusiasm for ballooning that began with the Mongolfier brothers and Charles in 1783, it was inevitable that the good and bad properties ofhydrogen would meet. The worst happened on 15th June 1785 when Pliatre de Rozier and an assistant, P. A. Ronaon, attempted to cross the English Channel in a hydrogen balloon carrying a small hotair balloon for altitude control. Thirty minutes into the flight the hydrogen ignited and the two men perished. Hydrogen's flammability was the underlying cause of the first air tragedy. Nevertheless, the attractiveness of hydrogen as a readily available buoyant gas was to outweigh the danger of flammability for 150 years of lighter-than-air flight. The Hindenburg disaster on 6th May 1937 in Lakehurst ended the air applications of hydrogen after 150 years.
The air application of hydrogen is the reverse of hydrogen's later role as a fuel, where its flammability is a major advantage and its low density the disadvantage that inhibits its use for flight in the atmosphere. Experiments using hydrogen as an engine fuel had already been carried out in 1820 by W. Cecil. These experiments were followed by investigations in car engines up until World War II and partly afterwards (Ricardo, Burstal, Erren). Various transport systems using hydrogen were tested, for example in the USA by. R. Billings or in Germany (Daimler Benz, BMW) in the last decade of the twentieth century. The potential of hydrogen as an aviation fuel was established in1957 with the lift-off of a hydrogen-powered B-57 Canberra twin-engine jet bomber. From 1963 onwards liquid hydrogen-liquid oxygen propelled rockets were launched. For the Apollo moon flights 12 x 103 m3 of liquid hydrogen was necessary to tank the Saturn carrier rockets. In 1970 Dr. Karl Kordesch built an alkaline fuel cell  (6kW, Union Carbide)/battery hybrid electric car based on an A-40 Austin sedan  (Fig. 2.5) and drove it for his own personal transportation needs for over three years.
The fuel cell was installed in the trunk of the car with compressed hydrogen tanks on the roof,leaving room for four passengers in the four door car. It had a driving range of 180 miles (300 km). Thus, Kordesch was the first person in the world to have produced and driven a practical fuel cell/battery electric car. In 1988 a triple-jet hydrogen powered Tupolew Tu-154 flew in the former Soviet Union. In 1994 Daimler-Benz demonstrated Necar 1 with a 50kW PEM fuel cell from Ballard and compressed hydrogen gas cylinders. In 1996 Toyota demonstrated RAV4 with a 10 kW PEM fuel cell and hydrogen stored in a metal hydride. Toyota discontinued the production of its RAV4 Electric Vehicle in 2003.
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