The number and variety of known compounrjs between proteins and small molecules are increasing rapidly and make a fascinating story. For instance, there are the compounds of iron, which is carried in our blood plasma by a globulin, two atoms of iron to each molecule of globulin held in a rather tight salt-lie binding? which is stored as ferric hydroxide by ferritin much as water is held by a sponge? and which functions in hemoglobin, four iron atoms in tight porphyrin complexes in each protein molecule. Or, there are many compounds of serum albumin, which was used during the war by many chemists, most of whom found at least one 6ew compound. This molecule, which has about a hundred carboxyl radicals, each of which can take on a proton, and about the same number of ammonium radicals, each of which PV" = RTm2(1 + Br2m2) 0.12 0.00

The method to be outlined is an-outgrowth of the treatment of gaseous systems used in the laboratory of the Massachusetts Institute of Technology. It may be regarded also as a quantitative development of the treatment of Hildebrand, although it disagrees with his ideas in some important details, or as a method of freeing the van Laar treatment from the inadequacies of the van der Waals equation. I n this paper is presented the simplest first approximation. Though its success is not astounding, it is hoped that the presentation will show some of the reasons for the limitations of so simple a theory, as well as indicate some of the possibilities of improvement. It will also serve as a comparative review of the different theories.The general method is to make all variations of composition1 and temperature a t such low pressures that the perfect gas laws are applicable, and to make the pressure variations a t constant temperature and composition (1). There is no obvious method of extending to liquids the treatment of free energy itself, but progress may be made through the energy. We may express the energy of a mole of liquid as Ul = Ua -u. Uo is the molal energy of the gas a t zero pressure, and we need not consider it further for it is a function only of the temperature, independent of the pressure, the state of aggregation, and of the composition unless there is a chemical change which persists a t zero pressure.

Sinclair and Robinson3 have developed the isotonic or isopiestic method4 so that it is more precise than any of the direct methods of measuring the chemical potential (or activity) of the solvent in which the temperature of measurement does not vary with the concentration. We have modified their method to give a still greater precision. We believe that our error is not greater than 0.1% of , the osmotic coefficient when the total molality, vm, is greater than one molal, and not greater than 0.001 in for smaller concentrations.In order to determine the chemical potential of the solvent, or any related quantity, from isotonic measurements, however, it is necessary to know it as a function of the composition for one solute. On the other hand, the method gives a way of comparing the measurements with different solutes. We have, therefore, made such measure-these substances.Cambridge, Mass.

Vapor pressures of 1.0-6.1 mol kg™1 aqueous sodium chloride were measured from 298 to 373K. The apparatus for measuring vapor pressures was modified to increase the convenience and the precision so that it could be operated by a single observer; the precision of measurement was ±0.002K or ±0.005 torr, whichever was larger. From these measurements, other vaporpressure and freezing-point measurements, and calorimetric enthalpies and heat capacities at 298K, 20 parameters of the modified Debye-Huckel-powerseries-in-the-molality equation were determined for aqueous sodium chloride. The equation was compared with measurements of the vapor pressure, the electromotive force of concentration cells, and thermal properties. The agreement was usually excellent.Previous static measurements of liquid-vapor equilibrium (33-35) indicated that the method is capable of even greater precision. Therefore, we reconstructed the cell and most of the auxiliary apparatus (9, 10). The changes which are of general interest were the replacement of the membrane null manometer by a liquid mercury manometer and the replacement of the air thermostat by an oil thermostat. The quantities to be measured were the equilibrium temperature and pressure, the total amount of each component, and the volume of the vapor phase.

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