142 PHYSIOLOGY 



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 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 deter- 

 mined by the dissociation which all these salts undergo in watery 



2 3 4 5 6 7 8 



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. 



solution. A dilute solution of sodium chloride contains, not the mole- 

 cule Nad, 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 electro- 

 lysis) 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 disso- 

 ciation, a 0'58 per cent, solution of NaCl would be isotonic or isosmotic 

 with a T8 per cent, solution of glucose. On account of the ionic disso- 

 ciation or ionisation, it is actually isosmotic with a glucose solution 

 of about 3 -6 per cent. 



INDIRECT METHODS OF MEASURING OSMOTIC PRESSURE. 

 Equimolecular solutions have the same osmotic pressures. Since the 

 osmotic pressure of a solution is therefore directly dependent on the 



