26 GENERAL BIOCHEMISTRY 



tension and gravity. Thus a porous solid whose surface is wet by water 

 becomes impregnated with water and may serve, like ordinary soil, as 

 a water reservoir. Movement of liquid through pores brought about 

 by surface forces is known as capillary rise or transfer and is generally 

 believed to play a role in the movement of biological fluids, especially 

 in plants and soils. 



Determination of surface tension is of value since the magnitude 

 of the various surface phenomena depends in turn upon the magni- 

 tude of the surface tension. A number of measuring methods are 

 available, all providing comparable results. These methods depend 

 upon the height a licjuitl reaches in a capillary of known diameter, 

 the si/e to which tlrops of liquid grow before breaking away from a 

 tip of fixed dimensions, or the force required to tear a standard ring 

 from the liquid surface. All these principles are widely used, and the 

 various equations are available in any textbook on physical chemistry. 



Viscosity 



Liquids are also characterized by their resistance to flow. This 

 resistance or internal friction is known as viscosity and has its origin 

 in the attractions between molecules. As a result it takes force to move 

 one layer of liquid past another, the magnitude of the force being a 

 measure of the resistance offered to flow. This force can be applied 

 in gravitational form, by means of gas pressure, or as a torque from 

 a rotating metal cylinder. Viscosity then equals the force applied 

 times the interval of time divided by the area of the shearing or 

 flowing surface. Since the latter usually cannot be readily measured, 

 it is customary to relate the time of flow of one liquid to another 

 in the same apparatus. If the absolute viscosity of the reference liquid 

 is known, the value for the first liquid is readily calculated. Methods 

 and calculations are described in elementary textbooks on physical 

 chemistry. 



Each liquid system possesses a characteristic viscosity depending 

 upon the sizes and the interactions of the particles present. Large 

 molecules and large intermolecular forces lead to high viscosities. 

 Likewise temperature has an effect, probably involving velocity effects 

 like those discussed on page 21 under vapor pressure and boiling. At 

 any rate, an increase in temperature commonly decreases viscosity, 

 presumably because the increase in molecular velocity helps overcome 

 intermolecular forces. 



The foregoing considerations apply in general to solutions in the 

 liquid state as well as to pure liquid. If the dissolved materials contain 

 relatively large molecules, they enhance the viscosity of the solution 



