222 



NUTRITION 



that of the solution. As to how the dissolved salt exerts its pressure 

 nothing is really known, although there are theories a-plenty. One 

 observer supposes that the dissolved salt exists in the interstices between 

 the molecules of the solvent in the state corresponding to a perfect gas, 

 and hence that the stronger the solution the greater its pressure through 

 a membrane toward a weaker solution. Another view (Poyn ting's) is that 

 "the phenomenon known as osmotic pressure arises from the molecules 

 of salt clinging to the molecules of water, and so diminishing the mo- 

 bility and therefore the rate of diffusion of the latter," each molecule of 

 salt completely impeding the movement of one molecule of water. 



Ions, the parts into which the molecules of many crystalloids dissociate, 

 act as regards the production of osmotic pressure just as would whole 

 molecules. It is supposedly on this account that a solution of an inor- 



FIG. 117 



Diagram to show osmosis through the vital membranes (cells of the spiderwort, Tradescantia) : 

 A. When the cell is put in a dissociated solution (electrolyte) having the same pressure as that 

 of the electrolytes of the cell biogen, no changes are apparent, and if in a less pressure solution 

 the cellulose wall prevents the expansion which else would be obvious (more water passing in 

 than electrolyte out). B and C. If immersed in a solution of stronger osmotic pressure, the 

 opposite movement occurs and the biogen is retracted. (Jones.) 



ganic acid or salt exerts more osmotic pressure than does an isotonic 

 (equally pressing) solution of organic acids or salts, for many of the latter 

 do not dissociate into ions. The solutions, then, whose molecules disso- 

 ciate most will exert the greatest osmotic pressure. Moreover, as 

 Arrhenius showed, when, for example, sodium chloride dissolves in water, 

 some of its molecules dissociate into sodium ions and chlorine ions. 

 The former are then bearers of positive "electrons" and the latter of 

 negative " electrons." By means of these electrons the solution conducts 

 electricity, and is called, therefore, an electrolyte. The greater the 

 conductivity of an electrolyte the greater is its osmotic pressure. A 

 solution of sodium chloride, therefore, injected into a mass of protoplasm 

 that dissociates little tends to leave it, owing to its greater osmotic 

 pressure, until a balance is found. It is on this principle, in part, that 

 the tissues maintain their accustomed normal composition and perform 

 some of their secretory functions. 



