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In the eighteenth century, Europe was full of inquisitive men attempting to unlock the secrets of the natural world, and Henry Cavendish was certainly one of the most eccentric. It is said that he was unable to bring himself to interact with women outside of his family at all, communicating with his female servants by written notes and sneaking around his own house using a specially constructed staircase so as to avoid his housekeeper. His isolation was so extreme that he often kept his experimental findings secret, not publishing or sharing his research with anyone. Such was the extent of this secretiveness that it was only many years after his death that the true breadth of his discoveries became apparent.

Cavendish was a follower of phlogiston theory – a widely held belief that had its roots in alchemy. The theory suggested the existence of an element thought to be contained within all combustible material, called ‘phlogiston’. By the middle of the eighteenth century the theory had been widely discredited, yet Cavendish continued to see worth in it, and attempted to incorporate it into many of his observations. To modern ears, this makes his terminology sound rather eccentric, but his contribution to our understanding of the natural world was extraordinary, not least in his early work on the chemical properties of water.

In a series of experiments, Cavendish produced and isolated a gas by reacting hydrochloric acid with metals such as zinc, iron and tin. In doing so, he became the first person to identify hydrogen in the laboratory. He referred to this new gas as ‘flammable air’ in his poetically named paper ‘Factitious Airs’, published in 1766. Cavendish went on to show that hydrogen reacted with another gas, which he termed ‘dephlogisticated air’, to produce water. This gas was oxygen. His experiments with flammable air eventually led him to the first determination of the composition of Earth’s atmosphere – one part dephlogisticated air (oxygen) and four parts ‘phlogisticated air’ (nitrogen). There is something quite instructive in Cavendish’s approach to science. Even though his devotion to the phlogiston theory was wayward, to say the least, he did not allow his theoretical prejudice to contaminate his experimental results. This is why he was able to make genuine discoveries while holding at least some views about his subject that were flat-out wrong. That is the mark of a great experimental scientist!

ELECTROLYSIS OF WATER

We owe the modern names for the elemental building blocks of water – hydrogen and oxygen – to Antoine Lavoisier, one of the greatest of the pioneering eighteenth-century chemists. Great though he undoubtedly was, however, he made a fundamental error in naming these two elements that persists to this day.

He named hydrogen, entirely appropriately, from the Greek ‘hydro’ (meaning water) and ‘genes’ (meaning creator). Oxygen, however, with its Greek root of ‘oxys’ (meaning acid), incorrectly suggests that oxygen is a component of all acids. It would have been more accurate to call hydrogen ‘oxygen’, in that the majority of common acid-base chemical reactions involve the transfer of protons, which are the nuclei of hydrogen. But Lavoisier’s names have stayed with us, so oxygen will forever be ‘the acid giver’, which it isn’t.

By 1804, the final elemental description of water was given in a paper by the French chemist Joseph Louis Gay-Lussac and the German naturalist Alexander von Humboldt. Together, they demonstrated that water consisted of two volumes of hydrogen to one of oxygen, and thus gave the world the most widely known of all chemical formulae: H₂O. If Lavoisier had got it right, we’d call water O₂H rather than H₂O. Such is history.