CHAPTER VI 

 LIBERATION OF ENERGY 



(4) SURFACE ENERGY 



" This Phaenomenon proceeds from a propriety wliich belongs to all kinds of fluid 

 Bodies more or less, and is caused by the Incongruity of the Ambient and included 

 Fluid, which so acts and modulates each other, that they acquire, as neer as is 

 possible, a spherical or globular form." Hooke, 1665.. 



(See also Chap. XIV. Muscular Contraction ; XV. and XVI. 

 Secretion and Excretion ; XVIII. Nerve Conduction ; XXVII. 

 Respiration.) 



Observation shows that the surface differs markedly in physical 

 state from the interior of a liquid. The surface between air and 

 w^ater, the air-water interface as it may be called, is able to with- 

 stand the application of a considerable distorting force without 

 rupture. This fact may be demonstrated in a variety of ways, e.g., 



(i) A sewing needle carefully placed on such a surface causes the 

 water to bend to accommodate its weight (Fig. 6). 



(ii) A cold tea-spoon or other object 

 ;e^=z^— -.^^#5^=^^^^ ^ lifted from the surface of a cold liquid 



^^ ^~_j£r^ will stretch some of the surface film 



J^i^z . adherent to it for quite a considerable 



~ '- — -• distance (Fig. 7). If the fluid and spoon 



are warm the film will break more readily. 



Fig. 6. — To show the depression x x" j. j. i i? 



of the surface of water when lucrcasc oi tcmperaturc lowcrs surface 



a needle is floated on it. i 



cohesion. 



(iii) Water creeps up the sides of beakers, is soaked up by blotting 

 paper, sponges, charcoal, capillary tubes, etc., against gravity. 



(iv) Water beetles walk safely over the surface of water, and 

 large heavy clams may suspend themselves from filaments anchored 

 to the undcr-surface of the water in aquaria. These examples 

 show that the film of water molecules at the surface possess a 

 remarkable tensile quality, surface energy, surface tension or 

 cohesion which in the case of a water-platinum interface is about 

 73 dynes per cm. at 20° C. 



How Measured. — There are four methods in common use for the 

 determination of either the absolute surface tension or its value 

 relative to water. 



44 



