function state2 = getStateAfterIsothermalProcess(state1, state2, gas)
  V = getVname(state1) ;
  equation.variableNames = [ "p" V "const"] ;
  equation.exponents     = [  1  1   -1   ] ;
  variableName = getUnknownVariableNameFromConstEquation(state2, equation) ;
  state2 = updateStateFromConstEquation(variableName, state1, state2, equation) ;
  state2.T = getVariableFromStateEquation("T", state2, gas) ;
end