48 SURFACE ENERGY 



will become + and the H 2 SO 4 surface layer -. The S.T. 

 decreases and some Hg will drop out. Alteration in this charge 

 by connecting the Hg to a negative electrode will produce an 

 increase of S.T. and an upward movement of the mercury. The 

 amount of movement depends on the difference of potential 

 existing between the Hg and the H 2 SO 4 , and may be used as a 

 means of measuring small differences of potential (see Fig. 5). 



A B C 



Via. 5. Form assumed by mercury globules under different electrical states. 

 B is a mercury globule immersed in dilute HoSO 4 . It assumes a more spherical 

 form A when connected with the negative pole of a battery, while connection to the 

 positive pole reduces surface tension as shown at C. 



II. Indirectly. Effect of solutes. Substances which when 

 fluid have a lower surface tension than water, lower surface ten- 

 sion when dissolved in water, and vice versa. Why this happens 

 is not known. A plausible explanation is that of ionisation ; 

 but if ionisation is the cause, why should weak organic acids- 

 only slightly dissociated have great lowering power, while 

 strongly dissociated inorganic salts have very little effect ? Very 

 few substances raise S.T., and that only to a very slight extent. 

 Most organic substances lower S.T. markedly, e.g. addition of 

 butyric acid to water to make a 1 per cent, solution lowers S.T. 

 from 75 to 35 dvnes. 



> 



A solid-liquid interface has always a lower S.T. than either 

 a liquid-liquid or a liquid-air interface. That is why water rises 

 in a capillary tube. 



It is obvious, therefore, that means exist for readily lowering S.T. 

 and so setting free energy. 



B. Solutions. 



The addition of a solute makes some alteration in the intrinsic 

 pressure of a fluid and necessitates a redistribution of energy. 

 Solutes are not universally dispersed throughout a solution. ' The 

 concentration throughout a fluid tends to be so adjusted as to 

 reduce the energy at every point in it to a minimum " (Gibbs- 

 Thomson Principle). Solutes that low r er surface tension are more 

 concentrated at the surface, while those raising the surface tension 

 are found in smallest amounts at the surface. This may be 

 deduced from the second Law of Energetics. (See Part II. for 

 demonstration of S.T. and for methods of estimation : adsorption 

 will be treated in Chap. VIII., Colloids.) 



