1. The investigations dealing with the properties of casein as an acid were reviewed.

2. The solubility of uncombined casein in water was measured at 5°C. and found to be 0.70±0.1 mg. of N per 100 gm. of water.

3. Robertson's solubility measurements of casein in bases at various temperatures were recalculated and found to agree well with more recent measurements.

4. By combining the observations of several investigators, as well as the author's measurements of the solubility of casein, in base, at various temperatures, the following conclusions were reached:

(a) The solubility of casein in base is affected by the temperature in a discontinuous manner.

(b) There exist two ranges of temperature, one, extending from about 21° to 37°C. and the other from about 60° to 85°C. where the solubility of casein in base is practically independent of temperature.

(c) From 37° to 60° the equivalent combining weight of casein rises from the value 2100 to about 3700 gm.

5. By comparing the values of base bound by 1 gm. of casein at the two temperature ranges with a constant, the value of base necessary to saturate the same amount of casein, it was found that the latter value is a common multiple of the former values, indicating the stoichiometric nature of the effect of temperature.

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