1911.] 



The Properties of Colloidal Systems. 



251 



of the non-ionised less than ra/400. The outer solution, if 90 per cent, 

 dissociated, would have a concentration in non-dissociated salt of 10 per cent, 

 of m/30, or ra/300. If its dissociation had been 87 per cent., the concentra- 

 tion of the non-ionised salt would have been m/400. The result may then 

 be taken as supporting Donnan's theory. 



The marked effect of foreign electrolytes in depressing the osmotic pressure 

 of Congo-red solutions described in my former paper is, for the most part, 

 due to this peculiar distribution of the salt between the two sides of the 

 membrane. In the third experiment of Table V the osmotic pressure 

 actually observed was 400 mm. Hg. That of the dye against water should 

 have been about 620 mm., that is 220 mm. higher than that found. But the 

 osmotic pressure of the m/180 sodium chloride in the outer fluid would be 

 206 mm., which must be deducted from the 620 mm., leaving 414 mm. 

 instead of 400 mm. observed. The osmotic pressure of the small amount of 

 the sodium chloride within the membrane, being less than 3 mm. Hg, may be 

 neglected. The difference between the observed and calculated values is 

 probably to be accounted for by aggregation of the dye induced by the 

 foreign salt, which would not be great, since the amount of this salt within 

 the membrane was so small. 



The Action of Carbon Dioxide. 



It remains to refer to the effect of the presence of carbon dioxide or other 

 acid in the outer fluid of the osmometer. 



Various observers have noticed the difficulty in obtaining anything like 

 permanent pressures with colloidal salts, such as those of caseinogen, similar 

 to those dealt with in the present paper. This fact is due to the access of 

 carbon dioxide in the air. 



In dealing with colloidal systems, which are essentially unstable, it is not 

 to be expected that really permanent osmotic pressures are to be obtained. 

 If the manometer readings in my experiments did not fall more than by 2 or 

 3 per cent, of their values in 24 hours I considered it justifiable to assume 

 that equilibrium was established between the two sides of the membrane. 

 Experiments showed that equilibrium between diffusible bodies took place in 

 the form of osmometer used in less than 24 hours. In the experiments of 

 Biltz and v. Vegesack* on benzo-purpurin the pressure fell from 9*6 to 5 cm. 

 of solution in 38 hours and to 073 in 84 hours. That it is possible to obtain 

 practically constant pressures for several days, if carbon dioxide is effectively 

 excluded, is shown by the following manometer readings from one of my 



* ' Zeits. f. physik. Chem.,' 1909, vol. 68, p. 367. 



