254 



The Properties of Colloidal Systems. 



Na" ions instead of two, gives double the osmotic pressure of the latter at the 

 same concentration, whereas it should be the same, on the view of the non- 

 activity of the diffusible ion. The suggestion is also disproved by theoretical 

 considerations. 



5. The curve expressing the ratio of the conductivity of Congo-red solutions 

 to their osmotic pressure is convex to the axis of abscissae when these are the 

 values of the conductivity. It is a straight line when expressing the relation 

 of osmotic pressure to molar concentration. 



6. The value of osmotic pressure per unit increase of conductivity rises 

 with concentration, forming an S-shaped curve. 



7. Difficulties are pointed out in the hypothesis of formation of complex 

 ions if these are supposed to contain both acid and base components. The 

 possibility of aggregated simple ions carrying the sum of the charges of their 

 components is suggested in order to explain the experimental results. 



8. Whatever may be the nature of the cation, that it is diffusible is shown 

 by the fact that the membrane is the seat of an electromotive force. The 

 sign and numerical values of this potential difference agree, within experi- 

 mental errors, with the equation deduced by Hardy. 



9. The distribution of a foreign salt, such as sodium chloride, between the 

 solution of the dye on one side of the membrane and water on the other side 

 is always such that its concentration is greater in the water. Numerically 

 the results agree with Donnan's view of equality of concentration of non- 

 ionised sodium chloride on both sides of the membrane, and not with that of 

 Biltz and v. Vegesack of equality of total diffusible ions. The effect of sodium 

 chloride on the osmotic pressure is chiefly due to this peculiarity of distribu- 

 tion, since its aggregating effect is relatively small. 



10. It is impossible to obtain even an approximately constant osmotic 

 pressure in the case of colloidal salts with a diffusible cation if carbon 

 dioxide be allowed access to the outer water of the osmometer. This effect is 

 shown to be due to the fact that the presence of H' ions in the outer fluid 

 allows Na* to escape by interchange of ions. The final result is the escape 

 of the greater part of the sodium from the interior and precipitation of the 

 acid salt. 



11. Congo red appears to obey the gas law so far as the effect of temperature 

 on the osmotic pressure of its solutions is concerned. 



The expenses of this research were defrayed by a grant from the 

 Government Grant Committee of the Eoyal Society. 



