William Thomson had observed the decrease in…

William Thomson had observed the decrease in temperature of a gas when it expands in a vacuum (later known as the Joule-Thomson effect) in 1852.

In a paper written the previous year, the British mathematical physicist and engineer had supported the theory that heat was a form of motion but admitted that he had been influenced only by the thought of Sir Humphry Davy and the experiments of James Prescott Joule and Julius Robert von Mayer, maintaining that experimental demonstration of the conversion of heat into work was still outstanding.

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 analyzing the results and suggesting further experiments.

The collaboration lasts from 1852 to 1856, its discoveries including the Joule-Thomson effect, sometimes called the Kelvin-Joule effect, and the published results do much to bring about general acceptance of Joule's work and the kinetic theory.