Many men have contributed to the establishment of this great principle, but it was actually discovered and proved by the labors of three or four. Although it was practically all done before the middle of the nineteenth century, its general popular recognition did not come until a quarter of a century later. The doctrine was first distinctly formulated by Robert Mayer, a German physician, who published in 1842 a suggestive paper on “The Forces of Inorganic Nature,” which, however, attracted little or no attention. Mayer had not approached the problem from an experimental stand-point, but at about the same time it was attacked most successfully from this side by a young Englishman, James Prescott Joule, son of a wealthy brewer of Manchester, England. Joule made the first really accurate determination of the mechanical equivalent of a given quantity of heat, a physical constant which Rumford had tried to measure, reaching only a rough approximation. Substantially Joule’s result was that the heat energy necessary to raise the temperature of any given mass of water one degree Fahr. is the equivalent of the mechanical energy required to lift that mass through a height of seven hundred and seventy-two feet against the force of the earth’s attraction; and, conversely, if a mass of water be allowed to fall through a distance of seven hundred and seventy-two feet under the action of gravity, and at the end of its motion be instantly arrested, the heat generated will suffice to raise its temperature one degree Fahr. Of such vast importance is this numerical coefficient that it has been called the golden number of the nineteenth century. Since Joule’s time it has been redetermined by several physicists, notably by Professor Rowland, of Baltimore, the general conclusion being that Joule’s number was somewhat, but not greatly, too small.