THE PERMEABILITY OF MEMBRANES 123 



of sea water, and regards the fact as being due to impermeability of the cell 

 membrane to ions. 



The fact that a membrane being impermeable to salts makes it a non-conductor is shown 

 in an interesting way in the method used by Morse and Horn (1901) in preparing copper ferro- 

 cyanide cells. By passing an electrical current through the membrane from copper sulphate 

 outside to potassium ferrocyanide inside, the imperfect places are filled up and the resistance 

 of the membrane gradually rises ; for example, in one case reported by Berkeley and Hartley 

 (1906, p. 487) the resistance of a membrane rose from 2,700 ohms to 300,000 ohms. 



Although the resistance offered by living cells to the passage of a current of 

 electricity is explained simply and satisfactorily by the existence of a membrane 

 which is impermeable to salts, it must not be overlooked that other explanations 

 have been advocated. It is very difficult or impossible to prove experimentally 

 that cells are complete non-conductors, owing to the practical impossibility of 

 removing all external electrolytes from the solution bathing them, except by 

 means which affect the normal state of the membrane. We cannot, therefore, make 

 the definite statement that cells are actual non-conductors, so that there is a 

 possibility that their high resistance may be due to the presence of electrolytically 

 dissociated colloids, enclosed in a membrane impermeable only to colloids. This 

 circumstance would, as we shall see more in detail later, oppose the passage of a 

 current in one direction entering the cell, and in the opposite direction on leaving 

 it, since the one ion is imprisoned. It may be objected to this view that the 

 presence of such colloids in the blood corpuscles has not been proved. 



If the electrolytes within the cell were combined with the cell-proteins, in the 

 form of non-dissociated salts, they would be non-conductors, since ions only can 

 convey a current. But there is no experimental evidence to warrant an 

 explanation of the facts of the case on such an assumption. Reasons have also 

 been given previously to show that adsorption is insufficient as an explanation, 

 since an adsorption compound exists only in presence of free electrolytes in the 

 liquid phase with which it is in contact. Free electrolytes must, therefore, be 

 present in the interior of living cells. Their existence in that situation has been, 

 in fact, demonstrated experimentally by Hober in two ways. 



The first of these (1910, 2) depends upon the fact that the capacity of a 

 condenser is increased when a conducting stratum is introduced into the dielectric 

 between the plates, and the amount of the increase is proportional to the con- 

 ductivity of the stratum. It will be clear that there is no question of ions 

 being able to leave the cells in such a case. By this method, the internal 

 conductivity of blood corpuscles, after repeated washing with cane sugar solution, 

 was found to be about the same as that of a decinormal potassium chloride solution. 

 The second method (1912, 2) is founded on an experiment by J. J. Thomson 

 (1895). A conducting body, placed in the axis of a coil of wire through which a 

 rapidly-alternating current is passed, diminishes the strength of this current by 

 damping the vibrations, and it does this in proportion to its own conductivity. 

 By this more sensitive method, the content of blood corpuscles in free electrolytes 

 showed itself to be equal to that of a O'l to 0'4 per cent, solution of potassium 

 chloride. The method was afterwards improved (1913) so as to require less 

 material, and at the same time to be increased in sensibility. Frog muscles were 

 also investigated by its means, and found to have an internal conductivity equal 

 to 0*1 to 0-2 per cent, sodium chloride. 



Comparing this number with the analyses of Fahr (1909), we note that a part of the salts 

 must be adsorbed on the colloid surfaces, or in chemical combination in some form other than 

 a dissociated salt, so that this part does not contribute to the conductivity, which is less than 

 what would be given by the total salts of Fahr's results. It is also of interest to note^that the 

 above value of the internal conductivity of muscle cells was obtained after six hours' soaking 

 in isotonic cane-sugar, so that the membrane had not allowed the electrolytes to escape from 

 the cell. 



It has been suggested by Roaf (1912, i. p. 146), as indicated above, that the 

 properties of a colloidal salt, in allowing a current to pass through a membrane in 

 one direction only, might account for the high resistance of cells, without the 

 necessity of a membrane impermeable to crystalloids. I showed indeed (1911, u. 

 p. 242) that if a salt, of which one ion only is in the colloidal state, be separated 



