THE PERMEABILITY OF MEMBRANES 119 



with the cell contents. Suppose this solution to be hypotonic. The cell will at first increase 

 in volume, as we have seen, whether the membrane is permeable to the solute or not. If 

 it is impermeable to the solute, this increase in volume is permanent. But, if the increase 

 in volume is not permanent, the cell must be more or less permeable. On the other hand, 

 it seems possible, if the membrane is inelastic, that a permanent increase in volume might 

 result from a hypotonic solution, even if the membrane is permeable to the solnte. The 

 first effect having been to dilute the contents until their osmotic pressure is equal to that 

 outside, while the membrane has allowed itself to be stretched without any elastic reaction, 

 there does not seem to be any force capable of returning the cell to its original volume. 

 This being so, caution is necessary in drawing conclusions, unless it is definitely known 

 that the membrane is elastic. 



Calculations made by Roaf (1912, i. p. 145) make it probable that equilibrium 

 between diffusible substances inside and outside the cell takes place with great 

 rapidity, so that it is possible that a process requiring seven days for equilibrium 

 in an osmometer with parchment paper might be complete in O001 minute in 

 the case of a cell, owing to the very large surface in proportion to volume in 

 this latter case. It is justifiable to assume, then, that osmotic equilibrium of 

 substances to which the membrane is permeable takes place practically almost 

 instantaneously. But, at the same time, in the case of partial permeability, 

 that is, if we regard the sieve as having only one hole in a thousand large 

 enough to permit the passage of the molecules of a particular solute, the rate of 

 diffusion of this solute through the membrane can be only about O'OOl of that 

 of another solute, which can pass through all the pores. 



Some experiments, made by Overton (1902) on the sartorius muscle of the 

 frog, serve to show the impermeability of cells to crystalloids. When placed in 

 ,0'7 per cent, sodium chloride, there was no change in weight, even in several 

 hours ; hence this solution is isotonic with the muscle (Overton, p. 1 29). Suppose 

 we add another substance to such a solution, if the muscle cells are impermeable 

 to it they must shrink in order to increase -their osmotic pressure by loss of 

 water. Overton adds methyl alcohol to the extent of 5 per cent. No effect is 

 produced; hence the cells are permeable to methyl alcohol (p. 167), for this 

 concentration of methyl alcohol raises the osmotic pressure of the salt solution 

 very considerably. If the substance added is slowly permeable, a mixture of 

 effects results. A muscle placed in a solution containing O35 per cent, sodium 

 chloride, and 3 per cent, ethylene glycol, i.e., a solution whose osmotic pressure 

 is equal to that of a 2 per cent, sodium chloride and therefore considerably 

 hypertonic, loses weight at first, as if impermeable to glycol, but afterwards 

 gains weight. The explanation is that the glycol can penetrate slowly, so that, 

 after a time, its concentration within and without the cell becomes equal and 

 the effect of 0'35 per cent, sodium chloride, which is hypotonic, remains alone 

 (p. 195). As to the third possible case, glucose when added produces the same 

 effect as sodium chloride of the same osmotic pressure, viz., permanent shrinking ; 

 hence the membrane is impermeable to it (p.- 224). 



There remains the possibility to be considered, whether the apparent im- 

 permeability to salts may not be sufficiently accounted for by the existence of 

 a membrane semi-permeable as regards colloids only, but permeable to electrolytes, 

 as appears to be the view taken by Roaf (1912, i. p. 145). Ostwald (1890) has 

 pointed out that it is sufficient for a membrane to be impermeable to one ion 

 only of an electrolytically dissociated salt in order that neither ion shall pass 

 through. Suppose, therefore, that we have a salt of a protein present, which 

 may be one with an acid to which the membrane is permeable, or a base of 

 similar permeability. If this salt is not hydrolytically dissociated, the fact that 

 the colloidal ion does not pass out will prevent the opposite diffusible ion from 

 doing so. But in such a case the colloidal salt must be present without any 

 colloidal salt of the other kind ; that is, we cannot have two colloidal salts, in 

 one of which the anion is diffusible and in the other the cation. 



For example, if there were a hydrochloride of a protein, and the sodium salt of a protein 

 together, the positive and negative inorganic ions would escape together, or sodium chloride 

 would diffuse out, without let or hindrance from electrostatic attraction on the part ( 

 colloidal ion. 



The hypothesis of a membrane impermeable only to colloids will not, therefore 



