CONSISTENCY OF COLLOIDS 65 



the different phases, (2) the surface friction of the internal surfaces, 

 (3) the surface tension of the internal surfaces and (4) the strength 

 of the electrical charge. To what extent the individual factors influ- 

 ence the internal friction is as yet unknown. 



We have, however, received valuable guidance from the study of 

 the effect of electrolytes. Especially remarkable is the parallelism 

 between swelling and internal friction. It depends, apparently, on 

 the fact that both phenomena are characterized by an increase, i.e., 

 multiplication, of the free surfaces. Thus, for instance, we see that 

 acids and alkalies which favor the swelling of gelatin also increase 

 the internal friction of albumin, because there probably occurs an in- 

 crease in the free surfaces of the albumin ions (see p. 153 et seq.)> 



Swelling and Shrinking. 



If a crystalloid (common salt, sugar) is thrown into water, it sub- 

 divides in it until finally it is completely dissolved; the particles of 

 salt or sugar lose their cohesion. A colloid (glue, wood) in contact 

 with water increases its volume, it swells; its particles retain their 

 cohesion. This property, however, is possessed only by hydrophile 

 colloids. 



The imbibition of water, that is, swelling, may either go on in- 

 definitely in the case of colloids, so that finally the particles are torn 

 asunder and a solution or sol is formed as in the case of albumin; or 

 the imbibition may reach its limit very rapidly, as in the case of wood. 

 Between these there are all sorts of transitions, e.g., glue. ; Only in 

 the case of gels is it usual to refer to swelling. In the organism, gels 

 having very slight ability to swell serve as covering and framework 

 (e.g., hicje, collagen, shells and wood). They are intended to retain 

 the outward form. The same is true of the supporting tissues of 

 the individual organs and even of the cells, vessel walls, the mem- 

 branes of the intestinal canal, connective tissues, the vascular bundles 

 of plants, cell membranes, etc. On the other hand, the cell content 

 possesses the ability to swell to a high degree. 



Every organ has a certain definite normal fluid content. A healthy 

 plant has a definite turgescence and the protoplasm of a healthy 

 animal a given degree of swelling; every abnormal change in this 

 signifies illness or even death. Without doubt swelling plays a very 

 important role in the case of many phenomena which have hitherto 

 been attributed to osmotic pressure. Indeed, the osmotic pressure 

 is only manifested completely in the presence of a semipermeable 

 membrane, whereas the ability to swell does not require the presence 

 of a membrane. Swelling may under some circumstances counter- 

 balance the osmotic pressure or even overcome it and concentrate 



