For James Prescott Joule (1818-1889), who came of a family of brewers and was early engaged himself in the brewing industry, was reserved, however, the distinction of discovering the exact relation between heat and mechanical energy. After having studied chemistry under Dalton at Manchester, he became engrossed in physical experimentation. In 1843 he prepared a paper _On the Calorific Effects of Magneto-Electricity and on the Mechanical Value of Heat_. In this he dealt with the relations between heat and the ordinary forms of mechanical power, and demonstrated that the mechanical energy spent "in turning a magneto-electrical machine is _converted into the heat_ evolved by the passage of the currents of induction through its coils; and, on the other hand, that the motive power of the electro-magnetic engine is obtained at the expense of the heat due to the chemical reactions of the battery by which it is worked." In 1844 he proceeded to apply the principles maintained in his earlier study to changes of temperature as related to changes in the density of gases. He was conscious of the practical, as well as the theoretical, import of his investigation. Indeed, it was through the determination by this illustrious pupil of Dalton's of the amount of heat produced by the compression of gases that one of the greatest improvements of the steam engine was later effected. Joule felt that his investigation at the same time confirmed the dynamical theory of heat which originated with Bacon, and had at a subsequent period been so well supported by the experiments of Rumford, Davy, and others. Already, in this paper of June, 1844, Joule had expressed the hope of ascertaining the mechanical equivalent of heat with the accuracy that its importance for physical science demanded. He returned to this question again and again. According to his final result the quantity of heat required to raise one pound of water in temperature by one degree Fahrenheit is equivalent to the mechanical energy required to raise 772.55 pounds through a distance of one foot. Heat was thus demonstrated to be a form of energy, the relation being constant between it and mechanical energy. Mechanical energy may be converted into heat; if heat disappears, some other form of energy, equivalent in amount to the heat lost, must replace it. The doctrine that a certain quantity of heat is always equivalent to a certain amount of mechanical energy is only a special case of the Law of the Conservation of Energy, first clearly enunciated by Joule and Helmholtz in 1847, and generally regarded as the most important scientific discovery of the nineteenth century.