1. This paper gives measurements of the influence of various electrolytes on the cataphoretic P.D. of particles of collodion coated with gelatin, of particles of casein, and of particles of boiled egg albumin in water at different pH. The influence of the same electrolyte was about the same in all three proteins.
2. It was found that the salts can be divided into two groups according to their effect on the P.D. at the isoelectric point. The salts of the first group including salts of the type of NaCl, CaCl2, and Na2SO4 affect the P.D. of proteins at the isoelectric point but little; the second group includes salts with a trivalent or tetravalent ion such as LaCl3 or Na4Fe(CN)6. These latter salts produce a high P.D. on the isoelectric particles, LaCl3 making them positively and Na4Fe(CN)6 making them negatively charged. This difference in the action of the two groups of salts agrees with the observations on the effect of the same salts on the anomalous osmosis through collodion membranes coated with gelatin.
3. At pH 4.0 the three proteins have a positive cataphoretic charge which is increased by LaCl3 but not by NaCl or CaCl2, and which is reversed by Na4Fe(CN)6, the latter salt making the cataphoretic charge of the particles strongly negative.
4. At pH 5.8 the protein particles have a negative cataphoretic charge which is strongly increased by Na4Fe(CN)6 but practically not at all by Na2SO4 or NaCl, and which is reversed by LaCl3. the latter salt making the cataphoretic charge of the particles strongly positive.
5. The fact that electrolytes affect the cataphoretic P.D. of protein particles in the same way, no matter whether the protein is denatured egg albumin or a genuine protein like gelatin, furnishes proof that the solutions of genuine proteins such as crystalline egg albumin or gelatin are not diaphasic systems, since we shall show in a subsequent paper that proteins insoluble in water, e.g. denatured egg albumin, are precipitated when the cataphoretic P.D. falls below a certain critical value, while water-soluble proteins, e.g. genuine crystalline egg albumin or gelatin, stay in solution even if the P.D. of the particles falls below the critical P.D.