The typical curve which has been secured for growth of organisms in varying hydrogen ion concentrations is platycurtic and bimodal. Jacques Loeb1 secured similar curves for the viscosity, osmotic pressure, potential difference, and swellability of gelatin and other proteins. He explained the bimodality of these graphs upon the basis of the Donnan's equilibrium, and arrived at the conclusion that the median minimum lying between the acid and the alkaline optima respectively is located at the isoelectric point of the protein in each case. In physico-chemical investigations, then, one seems quite warranted in regarding the median minimum of curves plotted along the pH scale as a sound criterion of the isoelectric point of the protein involved. A number of biologists, including Robbins2 and Pearsall,3 have undertaken to utilize this relationship in the interpretation of similar graphs obtained in the study of plants and animals, that is, concluding, for instance, that the median minimum for growth curves is a...