James Joule, William Thomson and the concept of a perfect gas

Abstract

In the early 1850s Joule and Thomson measured the cooling experienced by a flowing gas on passing an obstacle that caused a decrease in pressure. The mythical ‘perfect gas’, which conforms exactly to Boyle's and Charles's laws, would show no such cooling. They used their results to put the new theory of thermodynamics on a more secure foundation and to establish a practical route for converting the measurement of temperature on a gas scale to an absolute temperature based on the second law of thermodynamics. Their experiments were sound but their calculations were in error. Later in the century William Hampson and Carl von Linde independently devised a simple method of liquefying air based on Joule–Thomson cooling, but whereas Linde understood the theory, Hampson, and many chemists, confused the process with the cooling of a gas doing external work, which is an effect that would occur also with a perfect gas. James Dewar copied Hampson's process, without acknowledgement, in his work on the liquefaction of air and hydrogen at the Royal Institution. He, too, did not at first understand the theoretical basis of the apparatus. In the twentieth century the effect was exploited to investigate intermolecular forces, but it is now rarely used.