In his book, Theoretical Concepts in Physics, Malcolm Longair describes two experiments that Joule performed involving gas expansion (the discussion starts on pg. 217). In both experiments, there is a chamber containing gas, labeled A, and a chamber that is evacuated, labeled B. There is also a tube with a valve connecting A and B. Experiment 1: The whole setup is immersed in a bath of water and the valve opened. The gas spreads out evenly between A and B, with no temperature change of the water bath. Experiment 2: A and B are placed in their own separate water baths, and the valve is opened. The water surrounding A cools and that surrounding B warms. I understand why experiment 1 turns out the way it does, but I am not sure that I understand the result of experiment 2. Does the bath around B warm because gas molecules start bombarding the walls of the container, transferring kinetic energy to them? Does the bath around A cool because there are less molecules present to bombard its walls as some of the gas leaves? Is there a better way to think about this result?