128 PHYSIOLOGY 



OSMOTIC PRESSURE OF ELECTROLYTES. It will be noticed in the 

 last Table that the isotonic solutions of different salts contain these salts 

 in the proportion of their molecular weights, i.e. each solution contains the 

 same number of molecules of dissolved salt. For the term isotonic we 

 might therefore employ equimolecular. When, however, these salts are 

 compared with solutions of sugar, it is found that the osmotic pressures of 

 the salt solutions are double or nearly double those of equimolecular solu- 

 tions of sugar. The osmotic pressure of a sugar solution is equal to the 

 pressure which its molecules would exert if they occupied the same space 

 in a gaseous form. A dilute salt solution therefore acts as if every one of its 

 molecules were doubled. This deviation of salt solutions from solutions of sugar 

 is bound up with the power of the former to conduct an electric current. A 

 sugar solution conducts electricity little better than pure water. On the other 



FIG. 21. Diagram to illustrate Barger's method of determining osmotic 

 pressure. The upper figure shows the capillary tube with nine alter- 

 nate drops of cane sugar and the substance under investigation. 



hand, the smallest trace, of salt added to distilled water enormously increases 

 its conducting power. As Arrhenius has shown, this increase of the osmotic 

 pressure of a salt solution is determined by the 'dissociation which all these 

 salts undergo in watery solution. A dilute solution of sodium chloride con- 

 tains, not the molecule NaCl, but an equal number of the ions Na and Cl, Na 

 carrying a positive charge while the Cl ions carry a negative charge. As 

 regards osmotic pressure and various other properties, each of these charged 

 ions acts as a whole molecule. It is the existence of these ions which confers 

 on the salt solution the power of conducting electricity a power the exercise 

 of which is attended with a dissociation (an electrolysis) of the salt into its 

 constituent ions, the electro-positive ion being deposited at the negative 

 pole while the electro-negative ion is deposited at the positive pole. The 

 molecular weight of NaCl is 58-5. The molecular weight of glucose is 180. 

 If there were no dissociation, a 0-58 per cent, solution of NaCl would be 

 isotonic or isosmotic with a 1-8 per cent, solution of glucose. On account of 

 the ionic dissociation or ionisation, it is actually isosmotic with a glucose 

 solution of about 3-5 per cent. 



