84 



DISPERSE SYSTEMS 



would be high. However, definite statements Hke that cannot be 

 made generally about colloids. They are true of physiological 

 colloids, especially of those in gel form, but are not all applicable to 

 the suspensoids. 



(/) Ultrafiltration. Evidence tending to confirm the limits of 

 size found by optical methods of investigation is afforded by 

 experiments initiated by the classical series of ultrafiltrations of 

 Bechold. Membranes of known permeability are prepared, i.e. 

 the diameter of the pores is known, and the colloidal solution is 

 filtered through these by pressure. A series of filters is tried till 

 one is obtained which has the smallest pores which will allow the 

 colloid to pass through. Obviously the particles must be smaller 

 than the pores, and also, equally necessarily, they must be larger 

 than the next filter in the series. The sizes of particles obtained 

 in this way arc in reasonable agreement with the values obtained 

 from ultramicroscopic calculations. 



(g) Osmotic Pressure. Pure colloids in neutral water have a very 

 low osmotic pressure. This is just what one would expect when 

 one remembers that the osmotic pressure of a solution is related 

 to the number of particles dispersed in unit volume. The value of 

 the osmotic pressure of suspensoids is very small, and seems to vary 

 experimentally with the method of preparation of the colloid. 

 Hydrophilic colloids, both sols and gels, have a measurable 

 osmotic pressure, e.g. a 1-25 per cent, solution of pure egg albumin 

 (Lillie) gives a pressure of 20 mm. of mercury at room temperature. 

 It is very difficult to prepare pure hydrophilic dispersoids, because 

 of the way in which they retain crystalloids, and the presence of 

 these salts materially modifies the osmotic pressure of the colloid. 

 This modification is not simply additive as it would be if salts were 



