When the pressure of fluid changes without heat addition, 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. The adiabatic lapse rate of water is the rate at which its temperature changes with pressure at constant entropy S, and salinity. 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. The present paper develops a theory showing 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. The result shows that the traditional thermodynamic equations cannot be used to describe the adiabatic compression of substances because they are derived from the equation which describes heat exchange. The traditional equations predict that substances with negative thermal expansion absorb heat under compression, while numerous experiments show that they express heat. Result also shows that water absorbs heat when it has negative thermal expansion. As many substances with negative compressibility have recently been found, this explanation appears to be plausible. Therefore, the study suggests precise experiments in this low-pressure region. Taking the salinity of water into account does not change the results of the theory.

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