266 THE REALITIES OF MODERN SCIENCE 



Consider for example an illustration in the case of 

 surface tension which, we have seen, decreases with an 

 increase in temperature. If the surface of a liquid is 

 increased, what will be the nature of the effect on the 

 temperature of the liquid? The temperature must 

 change in such a direction as to increase the surface 

 tension and thus to resist the increase in surface area. 

 Hence, the temperature must decrease. From our con- 

 siderations of molecular potential energy (see page 221) 

 we arrived at the same conclusion. Thus, an in- 

 crease in surface means an increase of p.e. due to the 

 increased separation of the molecules, and hence a de- 

 crease in the k.e., that is, a decrease in the temperature. 



Suppose the temperature of a two-phase system, 

 e.g. water and ice, is kept constant, how will the equi- 

 librium be affected by the introduction of a soluble 

 solid, e.g. a salt? The salt dissolves, in part at least. 

 Its introduction increases the concentration of salt 

 molecules from the original value of zero. The change 

 which takes place in the mixture must then be of such a 

 character as to oppose this change in concentration. 

 The equilibrium between the ice and the water is, 

 therefore, altered in such a manner as to tend to reduce 

 the concentration of the salt solution. Hence the ice 

 melts so as to increase the amount of the solvent water. 



An interesting comparison has been suggested be- 

 tween the principle of Le Chatelier-Braun and the 

 principle in organic life which is illustrated by the 

 " faculty of accommodation." This is the faculty by 

 virtue of which external actions upon a living organism 

 produce changes which tend to increase its power of 

 resistance to such external actions. Of this phe- 



