EVERY PRACTICAL source of energy useful to man, other than water power, needs to be transformed to work via a fluid when not used in internal combustion engines or for heat as such. I n the heat engines which make use of a motive fluid as a medium for the transformation of heat energy into work, the theoretical efficiency is higher if the motive fluid is condensable over the major portion of the cycle, and if the ratio of boiling to condensing temperature is high. Except for the binary mercury-steam power cycle, the fluid used in all such condensing power cycles so far has been water.The most common source of high temperatures for power production in any desired location a t moderate cost has been chemical reactions. The highest temperature obtainable economically in this manner is about 3600' F. from the combustion of coal. I n recent years, heat energy is becoming available from nuclear reactions. This heat can in principle be produced a t much higher temperatures than might be obtained from chemical reactions. Nuclear power plants are currently being developed to make use of this energy. Sodium is being considered as a motive fluid for several such high-temperature power cycleg. Its physical and thermodynamic characteristics together with its relative abundance and low cost make it comparable and in many respects superior to mercury as a motive fluid for high-temperature power cycles.To design engines to use sodium as a motive fluid, values of its thermodynamic properties should be available in the form of a chart or tables. A critical examination of the literature on the thermodynamic properties of sodium leads to the following conclusions.There are inconsistencies among the various correlating equations, in spite of a wealth of data on the vapor pressure of sodium. Moreover, the data do not extend to temperatures high enough to meet the practical operating conditions of a prospective power cycle. Actually, except for two measurements of the normal boiling point of sodium (13, 30) and the very recent data of Makansi, Muendel, and Selke (24), the experimental vapor pressure points extend only to 1113.1" K., corresponding to P = 0.635 atm. (29).The data on the specific heat, the enthalpy, and the entropy of liquid and solid sodium along the saturation curve (8) extend to 1173" K. and appear to be very accurate.Spectroscopic data indicate that sodium in the vapor phase consists of both monomer and dimer species. There exists, on the other hand, some evidence in the literature that sodium liquid does not contain any of the diatomic species. The amount of dimer in the vapor phase is determined by the reversible reaction:There is considerable discrepancy in the literature between the numerical values of the heat of dimerization for the above reaction. Values ranging from -16,559 to -20,000 cal. per gram mole of Naz for the heat of dimerization a t the ground state have been reported.Recent calculations of the thermodynamic functions of the two species of sodium vapor by the methods of statistical mechanics (14) wer...

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