1909.] 



The Properties of Colloidal Systems. 



285 



as given by the kinetic theory, the osmotic pressure of a solution containing, 

 n molecules per unit volume is 



n x 40 x 10 -15 atmos. 



When deduced from the rate of fall of the particles in a gamboge suspen- 

 sion, assuming Stokes' formula to apply, and taking n to refer to particles, 

 the osmotic pressure works out to be 



n x 36 x 10" 15 atmos. 

 When deduced from the distribution of particles in a vertical column, after 

 attainment of equilibrium, the formula becomes 



n x 42 x 10~ 15 atmos. 



From my observations, determining the concentration of particles by direct 

 enumeration under the ultra-microscope, the formula becomes 



n x 44 x 10~ 15 atmos. 



Such close approximations to the theoretical value must be more than mere 

 coincidence. 



Eamsay and Senter* also concluded, from experiments on the density of 

 arsenious sulphide solution taken by different methods, that the particles 

 behave as if in true solution. 



On the other hand, it is evident that my experiments lend no support to 

 the theory according to which the osmotic pressure of a colloidal solution is 

 due, in some way not very clear, to ions associated with the colloidal 

 particles. It is difficult to understand how these ions can still exert their 

 osmotic pressure when forming part of a complex system, which must move 

 and act as a whole. This much may be said, congo-red gives an osmotic 

 pressure which is at its highest when foreign electrolytes are most effectively 

 excluded. This must be understood as in no way excluding, as the ultimate 

 source of the negative charge, electrolytic dissociation of the colloid itself. 



It is very doubtful whether electrolytes in the state of adsorption are 

 ionised at all. Euerf finds that the chlorine present in colloidal zirconium 

 hydroxide gives no reaction with silver nitrate. Similarly in the case of 

 ferric hydroxide, the chlorine can only be detected after destruction of the 

 colloid by nitric acid. 



The general conclusion to be drawn is, I think, that whether a body 

 present in solution be in the form of particles, molecules or ions, each of 



* B. A. Eeports, 1901. In 1905 (' Journ. Phys. Cheni.,' vol. 9, p. 319) Senter also made 

 the suggestion that Brownian movement in colloids is equivalent to molecular movement 

 in true solutions. 



t 'Zeits. f. Anor . Chem.,' vol. 43, pp. 83—93, 1905. 



