‘H Two O’ to ‘O Two H’

It was not until the late eighteenth century—over a hundred years after the discovery of phosphorus—that it was appreciated that both phosphorus and sulfur were actually elements. Prior to this time, it was thought that all matter was made up of four so-called elements: earth, air, fire, and water. The realization that this was not so centred on understanding that the air is actually composed of a number of different gases, and in particular, understanding what happens when things burn. The discovery that water could be broken down into, or indeed synthesized from, two simpler elementary substances started a chemical revolution in France. The fruits of this revolution are embodied in the very names we now use for these two components, hydrogen and oxygen. However, the path to enlightenment was tortuous, lasting over 200 years. At its peak at the end of the eighteenth century, chemists fell into two distinct camps—those for the new French chemistry, and those against it. Several different names were given to the gases before ‘hydrogen’ and ‘oxygen’ triumphed. As it turns out, one of these names is still based on an incorrect theory, and it might have been more appropriate if the names hydrogen and oxygen had been swapped around. From the sixth century BC, the ancient Greek philosopher Thales taught that water was the primary matter from which all other substances were formed. Perhaps this idea came from water’s ready ability to form solid ice, ‘earth’, or vapours and mists, ‘airs’. Other philosophers thought the primary substance was air; others still, fire. It was less common for earth to be thought of in this way, possibly, as Aristotle later wrote, because it was too coarse-grained to make up these fluids. In the fifth century BC Empedokles brought the four ‘elements’ together—earth, air, fire, and water—and for many centuries it was thought that these made up everything around us.