Much of the initial resistance to Joule's work stemmed from its dependence upon extremely
precise measurements. He claimed to be able to measure temperatures to within of a degree Fahrenheit (3 mK). Such precision was certainly uncommon in contemporary experimental physics but his doubters may have neglected his experience in the art of brewing and his access to its practical technologies. He was also ably supported by
scientific instrument-maker
John Benjamin Dancer. Joule's experiments complemented the theoretical work of
Rudolf Clausius, who is considered by some to be the coinventor of the energy concept. Joule was proposing a kinetic theory of heat (he believed it to be a form of rotational, rather than translational,
kinetic energy), and this required a conceptual leap: if heat was a form of molecular motion, why did the motion of the molecules not gradually die out? Joule's ideas required one to believe that the collisions of molecules were perfectly elastic. Importantly, the very existence of
atoms and
molecules was not widely accepted for another 50 years, though the
essential work on the existence of molecules, atoms and electrons was underway throughout the 19th and early 20th centuries, from that of
John Dalton through to
Ernest Rutherford. A collection of Dalton’s works was published in 1893, 49 years after his death. Although it may be hard today to understand the allure of the
caloric theory, at the time it seemed to have some clear advantages.
Carnot's successful theory of heat engines had also been based on the caloric assumption, and only later was it proved by
Lord Kelvin that Carnot's mathematics were equally valid without assuming a caloric fluid. However, in Germany,
Hermann Helmholtz became aware both of Joule's work and the similar 1842 work of
Julius Robert von Mayer. Though both men had been neglected since their respective publications, Helmholtz's definitive 1847 declaration of the
conservation of energy credited them both. Also in 1847, another of Joule's presentations at the British Association in
Oxford was attended by
George Gabriel Stokes,
Michael Faraday, and the precocious and maverick
William Thomson, later to become Lord Kelvin, who had just been appointed professor of
natural philosophy at the
University of Glasgow. Stokes was "inclined to be a Joulite" and Faraday was "much struck with it" though he harboured doubts. Thomson was intrigued but sceptical. Unanticipated, Thomson and Joule met later that year in
Chamonix. Joule married Amelia Grimes on 18 August and the couple went on honeymoon. Marital enthusiasm notwithstanding, Joule and Thomson arranged to attempt an experiment a few days later to measure the temperature difference between the top and bottom of the
Cascade de Sallanches waterfall, though this subsequently proved impractical. Though Thomson felt that Joule's results demanded theoretical explanation, he retreated into a spirited defence of the
Carnot–
Clapeyron school. In his 1848 account of
absolute temperature, Thomson wrote that "the conversion of heat (or caloric) into mechanical effect is probably impossible, certainly undiscovered" – but a footnote signalled his first doubts about the caloric theory, referring to Joule's "very remarkable discoveries". Surprisingly, Thomson did not send Joule a copy of his paper but when Joule eventually read it he wrote to Thomson on 6 October, claiming that his studies had demonstrated conversion of heat into work but that he was planning further experiments. Thomson replied on the 27th, revealing that he was planning his own experiments and hoping for a reconciliation of their two views. Though Thomson conducted no new experiments, over the next two years he became increasingly dissatisfied with Carnot's theory and convinced of Joule's. In his 1851 paper, Thomson was willing to go no further than a compromise and declared "the whole theory of the motive power of heat is founded on two propositions, due respectively to Joule, and to Carnot and Clausius". As soon as Joule read the paper he wrote to Thomson with his comments and questions. Thus began a fruitful, though largely epistolary, collaboration between the two men, Joule conducting experiments, Thomson analysing the results and suggesting further experiments. The collaboration lasted from 1852 to 1856, its discoveries including the
Joule–Thomson effect, and the published results did much to bring about general acceptance of Joule's work and the
kinetic theory. ==Kinetic theory==