The principal concern of this project is the transport of vapour across gas-filled cavities. For this to occur, there must be some gradient or difference in thermodynamic potential imposed by the boundary conditions. In any physical situation this is determined dynamically by the state of the surrounding media and the conditions at the interface are that certain quantities, such as the thermodynamic potential (Guggenheim 1959, p. 230) and flux of the vapour species, should be continuous. The vapour transport in the surrounding media depends in turn on the conditions imposed at its external boundaries, and so on ad infinitum. Thus, it is necessary to terminate the domain somewhere: the simplest choice is the boundary of the gas-vapour phase.