COLLOIDS 



65 



The size of drops of liquid falling from an orifice is dependent on sur- 

 face tension; the larger the drops, the greater the surface tension. If 

 the number of drops obtained by allowing a liquid to drop from a stand- 

 ard orifice in a given time is counted, we have a measure of the surface 

 tension. Account must of course also be taken of the specific gravity 

 of the liquid. The instrument used for this purpose is called a 

 stalagmometer (Fig. 18). Another method depends on the fact that 

 the height to which a fluid rises in a capillary tube is dependent on 

 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 determined for the other. 

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

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



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. 



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 



