1.2.2 Walls

Systems are bounded by walls. Walls determine what type of system one can have. Walls can be rigid or movable. They can be diathermal and allow heat to pass through them or be adiabatic and not allow heat to pass. Or they can be impermeable to matter or perhaps permeable to all or only some forms of matter.^1.1

Walls can have any combination of the three general types of attributes. A closed system, for instance, would have walls that are rigid, adiabatic, and impermeable, while walls that were movable, diathermal, and permeable would be imaginary boundaries prohibiting nothing.