132 REPORTS ON THE STATE OF SCIENCE.—1918. 
manner in which it is possible to explain the conduction along a 
nerve fibre on the basis of the disappearance of the electrical charge 
described below is explained (1915). 
II. Hlectromotive Phenomena. If a membrane is permeable to 
ions of one sign only, a Helmholtz double layer is established, such 
that the opposite sides of the membrane obtain opposite chaiges, 
owing to the ions held there. In other words, the membrane is 
polarized. It has been known for many years that muscle and nerve 
fibres show, on testing, that their outer surfaces have a positive 
charge. This can be most satisfactorily accounted for on the hypo- 
thesis that the cell-membrane is impermeable to certain anions, 
permeable to the corresponding cations. In the address to the 
Physiology Section of the British Association in 1915, I showed how 
this view explains the phenomena met with and may venture to 
repeat the paragraph here :— 
“Suppose that we lead off, te some instrument capable of detecting 
differences of electrical potential, two places on the outer surface of 
a cell having the properties referred to. It will be clear that we shall 
obtain no indication of the presence of the electrical charge, because 
the two points are equipotential and we cannot get at the interior of 
the cell without destroying its structure, But if excitation means in- 
creased permeability, the double layer will disappear at an excited spot, 
owing to indiscriminate mixing of both kinds of ions, and we are then 
practically leading off from the interior of the cell, that is, from the 
internal component of the double layer, while the unexcited spot is still 
led off from the outer component. The two contacts are no longer 
equipotential. Since we find experimentally that a point at rest is 
electrically positive to an excited one, the outer component must be 
positive, or the membrane is permeable to certain cations, impermeable 
to the corresponding anions. Any action on the cell such as would 
make the membrane permeable-injury—certain chemical agents and 
so on—would have the same effect as the state of excitation.” The 
peint of view taken here is practically identical with that of 
Bernstein (1913). Loeb (1915) has brought some objections to this 
view, based mainly on the fact that the application of salts to one of 
the places led off results in a change of the potential difference of 
such magnitude as to follow the well-known Nernst formula for 
concentration cells. I may point out that I have been able to show, 
both experimentally and by calculation (1911 and ‘General Physio- 
logy,’ pp. 648-650), that one can imitate the muscle cell by means of 
an osmometer, closed by a parchment paper membrane and filled with 
a solution of congo-red. This membrane is permeable to the sodium 
ions resulting from the electrolytic dissociation of the dye, but im- 
permeable to the anions. The only difference is in the sign of the 
charge on the inside and outside, the latter being negative in this 
case. The potential difference between the two sides, which can be 
modified or abolished by the addition to the outer liquid of a solute 
giving sodium or other cations, is found to be in accordance with the 
Nernst formula. In fact, a cell of the kind described is a model of 
the rationale of the electrode potential of the concentration battery. 
It seems that-there is then no real opposition between the results 
obtained by Loeb and Beutner (1912) and the hypothesis advocated 
