gaining the fellowship, he spent some time in the laboratory of the celebrated Henri Victor Regnault, at Paris; but in 1846 he was appointed to the chair of natural philosophy in the University of Glasgow. At twenty-two he found himself wearing the gown of a professor in one of the oldest Universities in the country, and lecturing to the class of which he was a first year student a few years before. Thermodynamics By 1847, Thomson had already gained a reputation as a precocious and maverick scientist when he attended the British Association for the Advancement of Science annual meeting in Oxford. At that meeting, he heard James Prescott Joule making yet another of his, so far, ineffective attempts to discredit the caloric theory of heat and the theory of the heat engine built upon it by Sadi Carnot and Émile Clapeyron. Joule argued for the mutual convertibility of heat and mechanical work and for their mechanical equivalence. Thomson was intrigued but sceptical. Though he felt that Joule's results demanded theoretical explanation, he retreated into an even deeper commitment to the Carnot–Clapeyron school. He predicted that the melting point of ice must fall with pressure, otherwise its expansion on freezing could be exploited in a perpetuum mobile. Experimental confirmation in his laboratory did much to bolster his beliefs. In 1848, he extended the Carnot–Clapeyron theory further through his dissatisfaction that the gas thermometer provided only an operational definition of temperature. He proposed an absolute temperature scale in which "a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T−1)°, would give out the same mechanical effect [work], whatever be the number T." Such a scale would be "quite independent of the physical properties of any specific substance." By employing such a "waterfall", Thomson postulated that a point would be reached at which no further heat (caloric) could be transferred, the point of absolute zero about which Guillaume Amontons had speculated in 1702. "Reflections on the Motive Power of Heat", published by Carnot in French in 1824, the year of Lord Kelvin's birth, used −267 as an estimate of the absolute zero temperature. Thomson used data published by Regnault to calibrate his scale against established measurements. In his publication, Thomson wrote: —But a footnote signalled his first doubts about the caloric theory, referring to Joule's very remarkable discoveries.