Joule concluded that U
does not change when a gas expands isothermally and therefore that πT = 0. His
experiment, however, was crude. In particular, the heat capacity of the apparatus was
so large that the temperature change that gases do in fact cause was too small to measure.
Nevertheless, from his experiment Joule had extracted an essential limiting
property of a gas, a property of a perfect gas, without detecting the small deviations
characteristic of real gases.