578 REVERSAL OF SIGN OF CHARGE 



ions are especially influential in determining the sign of charge of the 

 membrane. In alkaline solutions the OH ions are said to be adsorbed 

 by the membrane, while the mobile, watery stratum of the double 

 layer is formed by the cations ; in acid solutions the H ions are thought 

 to be adsorbed while the anions form the mobile stratum of the 

 double layer. When the OH ions are adsorbed by the membrane the 

 latter is negatively charged, when the H ions are adsorbed the mem- 

 brane is positively charged. This view meets a difficulty in the fact 

 that as a general rule membranes are negatively charged when in 

 contact with neutral water. The adsorption hypothesis meets this 

 difficulty with an additional assumption; namely, that in a neutral 

 solution the OH ions have a greater tendency to be adsorbed by a 

 membrane than the H ions. We should, however, be forced to 

 assume that the preferential adsorption of OH ions occurs also in 

 some cases in acid solutions, since we have seen that collodion mem- 

 branes (not treated with a protein) are negatively charged even in 

 strong acid solutions.^ Another difficulty was pointed out by Perrin 

 himself, namely that no other monovalent ion except the H and OH 

 ions were able to reverse the sign of charge of a membrane, and he 

 intimated that this might be due to the fact that the velocities of H 

 and OH ions are greater than those of any other ion. But if the 

 velocity determines the relative degree of adsorption of ions then 

 the H ions should be more readily adsorbed by a membrane in neutral 

 solutions than the OH ions. 



A second view, which is held chiefly by physicists, considers the for- 

 mation of an electrical double layer at the boundary of membrane and 

 water as a case of contact electricity, which may be influenced but 

 which need not be caused by ions. Lenard* has shown that when 

 very minute particles are torn off from the free surface of water the 

 minute particles are negatively charged while the water assumes a 

 positive charge. He concludes from this that at the surface of the 

 water there exists an electrical double layer the external stratum of 

 which is negatively charged, while the internal stratum is positively 

 charged. Since he was able to show that such a double layer exists 

 at the surface of water even in a vacuum, this double layer cannot 



3 Loeb, J., /. Gen. Physiol, 1919-20, ii, 255. 

 ^Lenard, P., Ann. Physik, 1915, xlvii, 463. 



