Vapor Pressure of Ammonia ammonia is equivalent to a change in the vapor pressm-e of about 2 mm, 12 mm, and 40 mm of mercury at -50^,0°, and +5o°C, respectively, or a percentage change in pressure of about 0.7, 0.4, and 0.25, respectively. The aim in the present experiments was to maintain temperatures constant to 0.01°C, or better, for very long time intervals. Platinum resistance thermometers were employed for the temperature measurements and temperatures were read to thousandths of a degree. II. PREVIOUS MEASUREMENTSThe measurements of various observers are given in Table i, which also includes for comparison, in the columns designated p calc. the final results obtained in the present investigation. Various determinations of the normal boiling point of ammonia are given in Table 2.

The change of po (pressure times volume of a constant mass of gas) with pressure or density at constant temperature and low pressures is treated as a characteristic physical quantit.y. Various methods of determining this quantity from available experimental data for several gases, including He, Ne, A, H~, N2, and O2, are examined with special reference to the reliability of the values at 0° C.Reliable values for these quantities are important for many purposes, such as correcting temperatures measured by means of gas thermometers to the thermodynamic scale and in the derivation of the absolute t emperature of the ice point,

Various thermal properties of petroleum products are given in numerous tables which embody the results of a critical study of the data in the literature, together with unpublished data obtained at the Bureau of Standards. The tables contain what appear to be the most reliable values at present available. The experimental basis for each table, and the agreement of the tabulated values with experimental results, are given. Accompanying each table is a statement regarding the estimated accuracy of the data and a practical example of the use of the data. The tables have been prepared in forms convenient for use in engineering.

Measurements of the specific heat at constant pressure of superheated ammonia vapor were made within the range of temperature and pressure ordinarily used in refrigeration, namely, in the temperature interval -15 to +150°C . and at various pressures ranging from 0.5 to 20 atmospheres.Incidentally a few meas urements of the Joule-Thomson effect were made in order to evaluate certain small correction terms.The method employed is a familiar one in calorimetry, that of continuous flow combined with electric heating of the vapor. The principle of this method is to observe the rise in temperature produced by a measured electric power added as heat to a steady stream of vapor flowing through the instrument at a measured rate. ♦ A flow calorimeter, which was developed for the particular purpose of making these measurements on ammonia, is briefly described.

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