When the pressure of a fluid changes without the addition of heat, the temperature of the fluid changes; the rate at which the temperature changes with pressure is called the adiabatic lapse rate. According to thermodynamic equations, the adiabatic lapse rate is positive if the thermal expansion coefficient is positive, and negative if this coefficient is negative. Experiments show, however, that the adiabatic lapse rate is also positive for substances with negative thermal expansion, although for water it is negative when it has negative thermal expansion. In the present paper, a theory is developed which shows that the adiabatic lapse rate must always be positive, but is negative for water because it has negative compressibility in that temperature–pressure region. Numerous substances with negative compressibility have already been identified.
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