In 1844, Joule used the apparatus shown to prove that "no change of temperature occurs when air is allowed to expand in such a manner as not to develop mechanical power." Two copper vessels were placed in a water bath whose temperature was accurately known. The first vessel contained air at an elevated pressure while the second vessel was initially evacuated. A valve connecting the two vessels was opened so that the air expanded from the first to the second vessel and the process terminated with pressure equilibrium between the two vessels. During the process no change in temperature of the water bath was recorded. Show that Joule's experiment is a verification of the conservation of energy.