It will prove useful to distinguish between two types of boundary that can separate
the objects. A boundary is diathermic (thermally conducting; ‘dia’ is from the Greek
word for ‘through’) if a change of state is observed when two objects at different
temperatures are brought into contact. A metal container has diathermic walls. A
boundary is adiabatic (thermally insulating) if no change occurs even though the
two objects have different temperatures. A vacuum flask is an approximation to an
adiabatic container.