The year 1783 was one of Watt's most fruitful years of the dozen which may be said to have teemed with his inventions. His celebrated discovery of the composition of water was published in this year. The attempts made to deprive him of the honor of making this discovery ended in complete failure. Sir Humphrey Davy, Henry, Arago, Liebig, and many others of the highest authority acknowledged and established Watt's claims. The true greatness of the modest Watt was never more finely revealed than in his correspondence and papers published during the controversy. Watt wrote Dr. Black, April 21st, that he had handed his paper to Dr. Priestley to be read at the Royal Society. It contained the new idea of water, hitherto considered an element and now discovered to be a compound. Thus was announced one of the most wonderful discoveries found in the history of science. It was justly termed the beginning of a new era, the dawn of a new day in physical chemistry, indeed the real foundation for the new system of chemistry, and, according to Dr. Young, "a discovery perhaps of greater importance than any single fact which human ingenuity has ascertained either before or since." What Newton had done for light Watt was held to have done for water. Muirfield well says: It is interesting in a high degree to remark that for him who had so fully subdued to the use of man the gigantic power of steam it was also reserved to unfold its compound natural and elemental principles, as if on this subject there were to be nothing which his researches did not touch, nothing which they touched that they did not adorn. Arago says: In his memoir of the month of April, Priestley added an important circumstance to those resulting from the experiments of his predecessors: he proved that the weight of the water which is deposited upon the sides of the vessel, at the instant of the detonation of the oxygen and hydrogen, is precisely the same as the weights of the two gases. Watt, to whom Priestley communicated this important result, immediately perceived that proof was here afforded that water was not a simple body. Writing to his illustrious friend, he asks: What are the products of your experiment? They are _water_, _light_ and _heat_. Are we not, thence, authorised to conclude that water is a compound of the two gases, oxygen and hydrogen, deprived of a portion of their latent or elementary heat; that oxygen is water deprived of its hydrogen, but still united to its latent heat and light? If light be only a modification of heat, or a simple circumstance of its manifestation, or a component part of hydrogen, oxygen gas will be water deprived of its hydrogen, but combined with latent heat.