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Published online by Cambridge University Press: 24 October 2008
When a mass of air, saturated with water vapour at room temperature θ1, is expanded adiabatically, the temperature falls to a lower temperature θ2. In general the resulting density of water vapour is greater than the equilibrium value at the temperature θ2 and condensation commences. This condensation causes heating of the gas and the temperature rises until finally at some temperature θ3 the density of uncondensed vapour is equal to the equilibrium vapour density at θ3. (θ1 > θ3 > θ2.) Condensation then ceases and the air rises slowly to room temperature by heat conduction through the walls of the apparatus.
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† Powell (loc. cit.) has investigated the dependence of γ on the partial pressures of air and water vapour. For an air pressure of 20 cm. and a temperature of 20° C., the resulting value of γ is 1.39. The maximum error thus introduced into the determination of the quantities required is 2%; for pressures greater than 20 cm. and temperature less than 20° C. it is smaller.