Sir James Prescott Joule (1818–1889) became one of the most significant physicists of the nineteenth century, although his original interest in science was as a hobby and for practical business purposes. The son of a brewer, he began studying heat while investigating how to increase the efficiency of electric motors. His discovery of the relationship between heat and energy contributed to the discovery of the conservation of energy and the first law of thermodynamics. Volume 1 of Joule's scientific papers was published in 1884. It is organised chronologically and reveals the range of Joule's interests and the development of his thought. Volume 2, published in 1887, contains papers which he co-authored with other noted physicists including Scoresby, Playfair and William Thomson, later Lord Kelvin. Joule's work, both individual and collaborative, was fundamental to the development of significant areas of twentieth-century physics.