160 PROTOPLASM 



Values of surface tension relative to water are satisfactory, 

 but it is better practice to have absolute values expressed in 

 terms of the unit of force the dyne, which is the force that will 

 produce in one second a change in velocity of a centimeter per 

 second in one gram. On this basis, we can define surface tension 

 as the tension exhibited fcy the free surface of liquids measured 

 in dynes per centimeter. The formula F = IT expresses surface 

 tension in terms of the total force F along a line of length I on 

 the surface of a liquid the surface tension of which is T. 



Some surface-tension values in dynes per centimeter are: 



Water = 75.6 at 0°C. Ethyl alcohol = 21 .7 at 20°C. 



Water = 72 . 8 at 20°C. Olive oil = 33 . 5 at 20°C. 



Water = 68 . 6 at 50°C. 



The absolute surface tension of a liquid can be measured by the 

 Maxwell frame. A film of, say, soap formed across a frame has 

 two surfaces so that a pull exerted upon it by a wire with attached 

 weight is sustained by two surface membranes. If the wire is 

 0.5 cm. long, then it pulls against 1 cm. of surface and measures 

 the membrane tension per unit of length. This /orce is expressed 

 in dynes. The work done is force times distance and is expressed 

 in ergs. An erg is the work accomplished by one dyne in one 

 centimeter. The force (72.8 dynes per centimeter for water at 

 20°C.) is the surface tension; the work (72.8 ergs per square 

 centimeter for water at 20°C.) is the free surface energy. 



Surface tension may be measured in still another way — by 

 comparing the height to which a liquid rises in a capillary tube 

 with that reached by water. Water wets glass; therefore it will 

 climb up the surface of the glass and in so doing form a curved 

 meniscus (Fig. 95). If the glass is a fine tube, the water will 

 climb within it, until the weight of the column of water just 

 balances the surface tension of the meniscus. The absolute 

 surface tension is expressed in dynes with the aid of the formula 



^ _ r-h-D-g 



where T is the surface tension; r, the radius of the tube; h, the 

 height to which the hquid rises; D, its density; and g, the value 

 of gravity. The height to which alcohol will climb in a glass 



