66 PHYSICOCHEMICAL BASIS OF PHYSIOLOGICAL PROCESSES 



surface tension (and inversely on the diameter of the capillary). The 

 difference in the heights to which two liquids rise in capillary tubes of 

 known bore permits us to compare their surface tensions, and if this 

 is known for one of the solutions, it can be determine4 for the other. 

 Besides existing between liquid and air, surface tension also exists at 



Fib. 18. Traube's stalagmometer. The surface tension is proportional to the number of 

 drops formed in a given time. The right-angled tubes are for thin liquids, and the straight 

 one for blood and other viscous fluids. 



the interface between two immiscible liquids, and at that between sus- 

 pended solid particles and liquid, as in colloidal solutions. Since, as 

 we have seen, the surface area (interface) is enormously increased in 

 these solutions, a very great surface energy is present, for this is equal 

 to the surface tension multiplied by the surface area. 



ADSORPTION 



The surface tension between liquid and air is lowered when organic 

 substances are dissolved in the liquid, but is slightly raised when inor- 

 ganic salts are dissolved. The degree of lowering varies markedly ac- 

 cording to the organic substance dissolved, being very pronounced with 

 bile salts, upon which fact the well-known (Hay) test for the presence 

 of bile in urine is based. Between liquid and liquid, as well as between 

 solid and liquid, the surface tension is always lowered T)y dissolving sub- 

 stances in the liquid. Now, at the interfaces between solid particles and 

 liquid there must be a local accumulation of free surface energy, which 

 will be equal to the surface tension multiplied by the surface (inter- 

 face) area. A constant tendency exists for such free energy to be de- 

 creased and, since dissolved substances have this effect, they will become 

 concentrated at the interface. This is known as the principle of Willard 

 Gibls, and it is of fundamental importance to the biochemist, because 



