The boiling point of a liquid becomes higher as the pressure increases. The difference between liquid density r L and vapor density r V at the equilibrium becomes smaller as the boiling point increases. An interesting quantity concerning the liquid–vapor coexistence curve is the tie-line mid-point 12 (r L1r V) of r L and r V in the temperature T–density r plane. It is referred to as a diameter. Experimental studies have shown that the diameter is almost linear for many simple fluids such as argon, krypton, oxygen, and nitrogen.1 This is referred to as the law of rectilinear diameter. Another interesting matter is the liquid–vapor coexistence curve scaled by the critical constants. It agrees with one another for these simple fluids.1 This is one expression of the principle of corresponding state.