CHAPTER VI 

 THE LIBERATION OF ENERGY 



(4) SURFACE ENERGY 



" This Phaenomenon proceeds from a propriety which 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, ](><>.">. 



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

 Secretion and Excretion ; XVI. Nerve Conduction ; XXVI. 

 Respiration.) 



A. Pure Liquids. 



Two forces act on molecules : 



(a) A repellent force kinetic, revealed in osmotic pressure, 

 vapour tension, etc. 



(b) A cohesion or attractive force Newton's " Gravity." 



" Every particle attracts every other particle with a force 

 inversely proportional to the square of the distance between 

 them." This gives rise within the liquid to intrinsic pressure, 

 whose magnitude we have no direct means of measuring, and 

 whose energy we cannot utilise. Why ? Because the various 

 tractative forces acting on each and every molecule within the 

 liquid neutralise one another. The attractions, except on the 

 surface layer, are uniform and cancel out. Consider a single 

 internal molecule. The tractative forces acting on it may be re- 

 solved into four components acting cyclically at right angles to 

 one another. It is obvious that these forces are paired. That, 

 at twelve o'clock is equal to and opposite to that at six, and 

 therefore ineffective. Similarly, the eastwards pull at three 

 o'clock is neutralised by the westwards pull at nine. In the 

 surface layer, matters are different. One component, that is the 

 force pulling downward at 6 o'clock, has no opposing force at 

 12 to stabilise the molecule. There is, therefore, a state of strain 



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