The recent improvements in the apparatus and methods for the determination of hydrogen-ion concentration have afforded a much more trustworthy method of determining the actual acidity or alkalinity of such materials than is obtained by color reactions, and this method is now being extensively used in the study of the reaction of active protoplasm. It must be kept in mind that protoplasm is an heterogeneous mass and not an homogeneous solution, so that it is not always possible to determine the actual conditions as to neutrality of different parts of the protoplasm of a single cell, for example. Hence, one of the best methods of determining the reaction which is favorable to the life and activity of any given type of protoplasm is to investigate the reaction of a liquid medium in which the cells live and grow; this plan being based upon the assumption that a cell is not likely to have a reaction different from that of the medium which is favorable to its growth. The results of all of the many investigations which have dealt with this problem point to the conclusion that the normal reaction for living protoplasm is either neutral or very faintly alkaline; but that it becomes acid when the cell is working in the absence of sufficient oxygen, and after the death of the cell. The first effect of a change in the reaction toward acidity of the protoplasm is a decrease in the rate of respiration of the tissue, while increased alkalinity stimulates respiratory activity. Whet carried to the point of actual acidity, the respiratory coefficient becomes negative, and the cell actually gives off carbon dioxide because of the stoppage of the synthetic processes.