88 



DISPERSE SYSTEMS 



separate colloids and crystalloids from a mixture of these two 

 constituents. If instead of keeping one side of the dialysing 

 membrane immersed in water, running water is substituted, or the 

 water is changed often, the colloid in the dialyser can be freed from 

 practically all the crystalloid mixed with it. Fig. 18 is an illus- 

 tration of Abel's vividiffusion apparatus, by means of which 

 crystalloids — salts, glucose, amino acids, etc., can be removed 

 from the colloids — albumin, globulin, fibrinogen, etc., of the cir- 

 culating blood. It consists of a number of collodion tubes in 

 parallel, which may be interpolated between the two ends of a cut 

 artery in an anaesthetised animal so that they are functionally 

 part of an intact circulation. Now, as have we seen, a diffusible 



substance will pass 

 out into the sur- 

 rounding water in 

 the glass con- 

 tainer, at a rate 

 depending on the 

 difference in the 

 concentration of 

 that solute on both 

 sides of the mem- 

 b r a n e (F i ck's 

 Law). If we want, 

 say, to study the 

 amino acid content 

 of the circulating 

 blood, all we have 

 to do is to arrange 

 matters so that our outer liquid starts with a concentration of no 

 amino acids and a concentration of all the other diffusible sub- 

 stances of blood equal at least to their concentration in the blood. 

 (i) Viscosity. It is obvious that some liquids offer a greater 

 resistance to stirring than others. Water and all true solutions in 

 water, even fairly concentrated ones, differ little from one another 

 in this respect. Even if small particles are suspended in the water 

 to form a suspensoid, or larger particles such as a precipitate of 

 barium sulphate in suspension, the additional resistance to shearing 

 is not very great. But when one comes to deal with hydrophilic 

 sols, and more so with gels, considerable force is required to push 

 a stirring rod through the liquid, i.e. the viscosity of hydrophilic 

 colloids is much greater than that of water, e.g. at 38° C. water has 

 a viscosity of 6-6 x 10~^ dynes per sq. cm., and blood serum about 

 twice that amount, viz. from 9-12 X 10~^ dynes per cm^. 



Fig. 18. — Abel's vividiffusion apparatus. 



