46 LIBERATION OF ENERGY 



3. Capillary Rise Method. The height to which a Hquid will rise 

 in a capillary tube held vertically on the surface is an inverse 

 measure of the surface tension of the liquid. (Part II., p. 518.) 



4. Air Bubble Method. A value for the surface tension of a 

 liquid may be obtained by measuring the amount of force necessary 

 to blow a bubble of air into it from a capillary tube. 



Ageing of Surfaces. — All these methods require the observance 

 of standard conditions. For instance, it takes time for a new 

 surface to attain its normal tension after it has been disturbed by 

 the placing of a ring on it, by the formation of a new drop, or by 

 forcing it out of a tube by a bubble of gas. In fact, the adjustment 

 of a surface may go on for hours, though the maximum change 

 takes place very rapidly. After the elapse of a minute an approxi- 

 mately normal value for surface tension may be obtained. Again, 

 the temperature at which the measurements are carried out is 

 important — the higher the temperature the lower the tension 

 developed, till at the critical temperature it reaches a mininnmi 

 value. That is, surface tension is a phenomenon with a negaiixe 

 temperature cocjjicieht. 



These facts may be explained by reference to the forces which 

 act on all molecules — in solids, liquids and gases. 



Two forces act on molecules : 



(«) A repellent force — kinetic, revealed in vapour tension, etc. 



{b) A cohesive or attractive force — Newton's " Gravity." 



The latter gives rise within the liquid to intrinsic pressure, 

 whose magnitude we have no direct means of measuring, and 

 whose energy we cannot utilise — because the various tractati\"e 

 forces acting on each and every molecule zvithin 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 in any plane may be resolved 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 downwards, has no 

 opposing upward force to stabilise the molecule. There is, there- 

 fore, a state of strain in the surface area. 



Orientation on Surfaces. — As the result of this state of strain the 

 molecules at the surface are arranged more regularly than those in 

 the body of the liquid. One may consider the internal molecules of 

 water as lying at random with their long axes in no particular 

 plane, while at the siu-face the long axes are practically parallel to 



