1. Two proteins of the globulin type, serum globulin and tuberin, and the protein of milk, casein, have been purified (a) of the other proteins and (b) of the inorganic electrolytes with which they exist in nature. The methods that were employed are described.

2. All three proteins were found to be only very slightly soluble in water in the pure uncombined state. The solubility of each was accurately measured at 25.0° ± 0.1°C. The most probable solubility of the pseudoglobulin of serum was found to be 0.07 gm. in 1 liter; of tuberin 0.1 gm. and of casein 0.11 gm. The methods that were employed in their determination are described.

3. Each protein investigated dissolved in water to a constant and characteristic extent when the amount of protein precipitate with which the solution was in heterogeneous equilibrium was varied within wide limits. The solubility of a pure protein is therefore proposed as a fundamental physicochemical constant, which may be used in identifying and in classifying proteins.

4. The concentration of protein dissolved must be the sum of the concentration of the undissociated protein molecule which is in heterogeneous equilibrium with the protein precipitate, and of the concentration of the dissociated protein ions.

5. The dissociated ions of the dissolved protein give a hydrogen ion concentration to water that is also a characteristic of each protein.

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