120 PRINCIPLES OF GENERAL PHYSIOLOGY 



explain the semi-permeability of the cell to neutral salts. We have seen above 

 that there is no satisfactory evidence of combination between proteins and 

 such salts, and, moreover, the hypothesis in question leaves the impermeability 

 to glucose unaccounted for. Glucose does not form a compound with proteins 

 of the kind required, and according to Asher (1912) exists free in the blood. 



A further difficulty lies in the high osmotic pressure in certain cells ; to obtain a 

 of 11 atmospheres, a half molar solution is necessary, and when we remember that tin- 

 molecular weight of proteins is about 2,000, we see the impossibility of such a solution. The 

 total solid content of cells is only about 20 per cent., ana of young, growing, cambium still 

 less. Substances of small molecular weight only can give the observed osmotic pressure. 



The hsematocrite (Hedin, 1891), as applied to problems in permeability (Hober, 

 1910), is a practical use of the facts described in the preceding section. 



2. We pass on to discuss some facts relating to the distribution of crystalloids 

 between the cell and the surrounding medium, which necessitate the presence of a 

 membrane impermeable to crystalloids. These facts are of interest in other ways. 



The red blood corpuscles of the rabbit contain much more potassium than the 

 plasma which bathes them, and no sodium at all, according to the analyses of 

 Abderhalden (1898, p. 100). Thus : 



Plasma. Corpuscles. 



Potassium 0-259 5-229 I 



r, ,. A A 4n > per thousand. 



Sodium 4-442 ) r 



Such relations are impossible to account for except on the assumption of a 

 membrane impermeable to sodium and potassium, unless these substances are 

 combined with the colloids in an irreversible, non-dissociable, manner. It is easy 

 to show, moreover, that the salts of blood serum readily pass through a membrane 

 of parchment paper, which is impermeable to colloids, since they are frequently 

 removed in this way. If the membrane of the rabbit's blood corpuscles were 

 impermeable only as regards colloids, sodium salts from the serum must inevitably 

 pass through. 



It is true that, under certain conditions, as was found by Donnan (1911) and by myself 

 (1911, ii. p. 249), independently, there may be different concentrations of a freely-diffusible 

 salt in equilibrium within and without a membrane of parchment paper. This fact is brought 

 by Roaf (1912, i. p. 145) in support of the opinion that a membrane impermeable to 

 electrolytes is unnecessary, so that it must be considered briefly. Take the case of the sodium 

 salt of a protein or of Congo-red, in solution inside a membrane of parchment paper. As long 

 as water only is present on the other side of the membrane, the sodium ions cannot escape 

 further than the position in which their osmotic pressure is balanced by electrostatic 

 attraction to the opposite, colloidal, ion inside. A Helmholtz double layer is formed, the 

 sodium ions being outside. Now it is not to be supposed that the same individual ions are 

 always present in this double layer ; a perpetual interchange is going on between them and 

 those present in the body of the solutions. Moreover, since their position is due solely to 

 the fact of their possessing a positive charge, it is clear that if any other cations are in a 

 position to interchange with them, the process will take place. This state of affairs will 

 exist if any salt, say potassium chloride, is present in the outer solution. The external 

 component of the double layer in such a case will consist of both K' and Na* ions in relative 

 proportion, according to their respective concentrations in the solutions, and ultimately this 

 same proportion will be established throughout both solutions, whatever the absolute con- 

 centration of the ions therein. This fact was pointed out by Ostwald (1890, p.- 714) as 

 applying to the copper ferrocyanide membrane and found experimentally by W. A. Osborne 

 (1906) in the case of salts of caseinogen, or soaps within a parchment paper membrane, and 

 by myself in that of Congo-red or of serum proteins in similar conditions. Although the 

 ratio of the concentrations of the diffusible salts is the same on both sides of the membrane 

 in such cases, as already remarked, the absolute concentration is greater on that side 

 containing the colloidal solution. This fact seems to be due to the necessity that the 

 concentration of non-dissociated salt must be equal on both sides ; there are, in fact, so far 

 as one can see, no forces present capable of making possible a different concentration of 

 electrically-neutral, freely-diffusible, substances. If, then, we have say sodium chloride in 

 decimolar solution on the outside, and the sodium salt of Congo-red inside, assuming 10 per 

 cent, of the sodium chloride undissociated, this concentration of undissociated moleeules 

 must be the same inside ; this cannot lie the case if the total concentration of the chloride 

 is the same on both sides, since that inside will be less dissociated than that outside, owing 

 to the presence of the dye salt with an ion (Na~) common to both salts. This explanation of 

 the unequal distribution of sodium chloride on the two sides of a membrane applies also if 

 the diffusible salt placed outside has not, to begin with, an ion in common with the colloidal 

 salt, say potassium chloride, because, as pointed out above, after equilibrium is attained, there 

 will be present both inside and outside all the kinds of the diffusible ions of the system. This 



