18 COLLOIDS IN BIOLOGY AND MEDICINE 



Expressed in general terms, this means that a fluid 2 spreads itself 

 on the free surface of a fluid 1, if 



o"i = surface tension of fluid 1 acting against air. 

 cr 2 = surface tension of fluid 2 acting against air. 

 cri/2 = surface tension of fluid 1 acting against fluid 2. 



Similarly, a fluid 3 spreads over the common boundaries of two 

 fluids 1 and 2, whenever 



0"l/2 > 02/3 03/1. 



This phenomenon is of the greatest biological interest, because it 

 follows that many fluids must spread out on the boundaries of other 

 fluids or solid bodies and form films, and further, that solid particles, 

 suspensions and colloids must collect on surfaces, according to con- 

 ditions. The diminution of surface tension between two surfaces is 

 the forerunner of mixing or solution; there exists no surface tension 

 between two readily miscible fluids. We shall discuss the distribu- 

 tion of dissolved substances on interfaces more fully on page 33 in 

 the discussion of surface pellicles. We shall see that numerous 

 organized structures and indeed the movement of protoplasm and 

 of the lower animals are derived from this phenomenon of surface 

 tension. It is the monumental service of G. QUINCKE that he showed 

 the connection between the purely physical processes and the phe- 

 nomena of animate nature. 



The surface tension of solids is deduced from the fact that finely 

 divided particles are more easily and rapidly dissolved than coarser 

 ones; it also explains the fact that artificial hydrophobe colloids are 

 produced only in a dispersing medium in which they are wholly in- 

 soluble (see p. 73). The slightest solubility permits them to pass 

 from the dispersed phase into coarser particles. Only to a limited 

 extent is it possible directly to measure the surface tension of solid 

 bodies. ROENTGEN measured the a- for rubber /air and rubber/water 

 and TANGL * tested a new method on the interfaces of rubber/water 

 and paraffin/water. The basis for the method is that a tube of the 

 substance to be tested (rubber or paraffin) undergoes a change in 

 shape when it is plunged from the air into a fluid (water). 



Interfaces of Solutions. Whenever substances are dissolved in 

 one or both phases, there is usually a difference between the con- 

 centration on the surface and on the interior. These changes in 

 concentration at the surface are termed adsorption. A substance be- 

 comes concentrated upon the surface if it reduces the surface tension. 

 This is the most usual adsorption phenomenon. Only a few inor- 



