Many different explanations of the origin of the electric current which develops when the protoplasm is stimulated into activity have been suggested; but none of them have, as yet, any experimental confirmation. The most that can be said is that whenever any stimulus excites the protoplasm into activity, there is instantly developed in it an electrical disturbance, which continues as long as the action is in progress. Recent investigations, which have shown that there is a direct relation between many of the vital processes of protoplasm and the ratio of the electrolytes which it contains, particularly the ratio of sodium and potassium to calcium, would seem to indicate that the development of the electrical disturbance is a direct result of variations in the proportions of the salts of these metals, either brought about by, or themselves causing, changes in the permeability of the protoplasm, following the stimulus which determines the nature of the activity which it is to undergo. But there is as yet no indication concerning the mechanism by which this stimulation, with its resultant electrical phenomena, is transmitted to the protoplasm and accomplishes its characteristic effects. ACIDITY OR ALKALINITY OF PROTOPLASM The preceding sections of this chapter have dealt almost exclusively with the physical properties of protoplasm; including the phenomena of solution, ionization, surface boundary effects, and electrical disturbances, and their probable effects upon the chemical reactions which constitute its biological activities. It is necessary now to consider another phase of the physical chemistry of protoplasm, namely, its chemical reaction, whether acid, alkaline, or neutral, the effects of variation of this condition upon the activity of the protoplasm, and the mechanism by which it tends to preserve its own proper reaction in this respect. The earlier methods of investigation of the chemical reaction of protoplasm were all based upon its color reactions to various staining agents. These sometimes led to erroneous conclusions, because of the effects of the staining agent itself upon the tissue; some stains are poisonous and result in the death of the protoplasm, others do not easily penetrate the semi-permeable colloidal mass, others are themselves changed by the oxidizing or reducing action of the protoplasm, etc. Again, colloidal adsorption effects often lead to the so-called "capillary segregation" of added staining materials. So that this method of study must be used with great care, or wholly erroneous conclusions will be reached, and many of the earlier reports have subsequently been found to be incorrect.