1. Experiments on anomalous osmosis suggested that salts with trivalent cations, e.g. LaCl3, caused isoelectric gelatin to be positively charged, and salts with tetravalent anions, e.g. Na4Fe(CN)6, caused isoelectric gelatin to be negatively charged. In this paper direct measurements of the P.D. between gels of isoelectric gelatin and an aqueous solution as well as between solutions of isoelectric gelatin in a collodion bag and an aqueous solution are published which show that this suggestion was correct.
2. Experiments on anomalous osmosis suggested that salts like MgCl2, CaCl2, NaCl, LiCl, or Na2SO4 produce no charge on isoelectric gelatin and it is shown in this paper that direct measurements of the P.D. support this suggestion.
3. The question arose as to the nature of the mechanism by which trivalent and tetravalent ions cause the charge of isoelectric proteins. It is shown that salts with such ions act on isoelectric gelatin in a way similar to that in which acids or alkalies act, inasmuch as in low concentrations the positive charge of isoelectric gelatin increases with the concentration of the LaCl3 solution until a maximum is reached at a concentration of LaCl3 of about M/8,000; from then on a further increase in the concentration of LaCl3 diminishes the charge again. It is shown that the same is true for the action of Na4Fe(CN)6. From this it is inferred that the charge of the isoelectric gelatin under the influence of LaCl3 and Na4Fe(CN)6 at the isoelectric point is due to an ionization of the isoelectric protein by the trivalent or tetravalent ions.
4. This ionization might be due to a change of the pH of the solution, but experiments are reported which show that in addition to this influence on pH, LaCl3 causes an ionization of the protein in some other way, possibly by the formation of a complex cation, gelatin-La. Na4Fe(CN)6 might probably cause the formation of a complex anion of the type gelatin-Fe(CN)6. Isoelectric gelatin seems not to form such compounds with Ca, Na, Cl, or SO4.
5. Solutions of LaCl3 and Na4Fe(CN)6 influence the osmotic pressure of solutions of isoelectric gelatin in a similar way as they influence the P.D., inasmuch as in lower concentrations they raise the osmotic pressure of the gelatin solution until a maximum is reached at a concentration of about M/2,048 LaCl3 and M/4,096 Na4Fe(CN)6. A further increase of the concentration of the salt depresses the osmotic pressure again. NaCl, LiCl, MgCl2, CaCl2, and Na2SO4 do not act in this way.
6. Solutions of LaCl3 have only a depressing effect on the P.D. and osmotic pressure of gelatin chloride solutions of pH 3.0 and this depressing effect is quantitatively identical with that of solutions of CaCl2 and NaCl of the same concentration of Cl.