THE MEMBRANES. 79 



The process consisted in placing two porous cups, partly filled with 

 water, side by side in a solution of the salt, and electrolyzing with the 

 anode in one cup and the cathode in the other. At times the pores of 

 the anode cup became filled with a deposit of peroxide, and it was 

 noticed that whenever this occurred the volume of the contents of that 

 cell seemed to increase more rapidly than could be accounted for by 

 any probable "electrical endosmose" of the solvent. It was imme- 

 diately suspected that the deposit of peroxide in the porous wall was 

 playing the part of a semi-permeable membrane and, upon further 

 investigation, the suspicion was proved to be well founded. The pro- 

 cess by which the semi-permeable peroxide was deposited in the pores 

 of the cell and that by which osmotic membranes are now made differ 

 radically; but the mere fact of having formed one active membrane by 

 electrolytic means sufficed to suggest the practicability of employing 

 electricity for the production of all semi-permeable membranes. 



The work of Pfeffer was unique and brilliant, and its consequences 

 have been far-reaching and beneficent. The writer, after fourteen 

 years of activity in the same difficult field, has more reason than any 

 other to appreciate its great merit and less justification than any other 

 for detracting from its value. This statement of attitude is made with 

 a view to disarming any possible suspicion of careless criticism on the 

 part of the writer, when he states, on the basis of his own experience, 

 that none of the pressures recorded by Pfeffer could have been the maximum 

 pressures of his solutions. 



The final step in the preparation of Pfeffer's cells for quantitative 

 measurements was by means of a solution containing 3 per cent of 

 potassium ferrocyanide and 1.5 per cent of potassium nitrate. The 

 cells were filled with this solution, closed, and placed in 3 per cent solu- 

 tions of copper sulphate. Pfeffer states that the pressure subsequently 

 developed was usually something more than 3 atmospheres. If we 

 add the excess of the ferrocyanide's pressure (over that of the copper 

 sulphate) to the true pressure of the nitrate, the total pressure which 

 should have been developed is something more than 6.5 atmospheres. 

 The unavoidable conclusion is that the membranes did not perfectly 

 retain the solute, and that the subsequent measurements were under- 

 taken with defective cells. 



In order to show that the osmotic pressure of solutions obeys the law 

 of Boyle for gases, van't Hoff cites the pressures of the 1, 2, 4, and 6 per 

 cent solutions of cane sugar which were obtained by Pfeffer. These 

 pressures are strikingly proportional to the concentration, which is the 

 form of the law as applied to solutions. There was, at that time, and for 

 many years thereafter, no reason known why this evidence should not 

 be accepted at its face value, and its validity appears never to have been 

 questioned; accordingly one finds it repeated and emphasized in every 

 presentation of the subject of osmotic pressure from 1887 to the present 



