FOR centuries scientists believed that water was an element and indivisible. Three men, Cavendish, Watt and Lavoisier, have claimed the credit for the important discovery that it is a compound of oxygen and hydrogen. For about seventy years a controversy raged over the merits of their respective claims. The result may be briefly summarised.

It is necessary first to understand the terms then in use. All gases were referred to as air. But the properties of air varied. There was supposed to be an invisible substance, called phlogiston, the principle of fire, which was contained in all inflammable bodies and was given off when they burned. Air deprived of its phlogiston, " dephlogisticated air," was what we know as oxygen. " Inflammable air " covered all gases that will burn, including hydrogen. The latter was sometimes distinguished as " the inflammable air from metals."

In I776 two chemists, Warltire and Macquer, independently observed that when inflammable air is burnt in ordinary air, water is deposited. Priestley repeated the experiment, but no use was made of the observation. Cavendish heard of it, realised its importance and, in the summer of I78I, conducted a series of experiments in which he produced water by exploding a mixture of oxygen and hydrogen in a closed vessel. He reported these experiments to Priestley, who tried to repeat them, and in turn reported his efforts to Watt.

Watt jumped to the conclusion that water is a compound, consisting of dephlogisticated air and inflammable air, or phlogiston, and expressed this theory in a letter written to Priestley on April List, I783, which he asked him to send to the Royal Society together with the account of his (Priestley's) experiments. Watt's letter was not made public, because Priestley threw doubt on the theory, and Watt asked that it might be held back until he had made further investigations. But several scientists, including Cavendish, saw it. Meanwhile Cavendish concluded his experiments and, on January 15th, I784, read a paper to the Royal Society in which he put forward a similar theory of the composition of water, using the same terms and making no mention of Watt.

Watt's guess that water is a compound was certainly original, and it was a brilliant piece of intuition. But it was based on the experiments of Cavendish, reported to him via Priestley. His view of the nature of the ingredients was false, for by " inflammable air, or phlogiston " he meant, not hydrogen, but any inflammable gas. Cavendish, on the other hand, was, in a sense, right. For though he spoke vaguely of " phlogiston," he had in his experiments always used hydrogen and knew that no other gas would serve. It remained for Lavoisier to complete the discovery by showing that phlogiston was a myth, and that hydrogen is a perfectly definite and distinct gas.

Watt thought that it was his letter that suggested to Cavendish the idea that water is a compound. The fact that Priestley was surprised when he heard Watt's theory might be taken to indicate that he had had no hint of anything similar from Cavendish, with whom he was in close touch. But Priestley often misunderstood what Cavendish was doing, and blundered badly when he tried to imitate him, so his evidence is unreliable. It is incredible that a man of Cavendish's character should have repudiated so great an obligation to a man whom he honoured as a scientist and valued as a friend. We may assume, therefore, that he understood the meaning of his own experiments without the help of Watt.