54 LIBERATION OF ENERGY 



nature of the surfaces to which they are exposed, but by the 

 chemical nature of the surfaces, Kaohn and ferric hydroxide will 

 not adsorb even a trace of tri-butyrin or acetone, or of any of the 

 higher alcohols, or, in fact, of any capillary active substance. 



The amount of any solute adsorbed depends on its concentration 

 and temperature, as well as on the nature of the force causing the 

 adsorption. If the volume and temperature of a solution are kept 

 constant while the quantities of adsorbent and of material to be 

 adsorbed are varied, it is found that when adsorption equilibrium 

 has been established, the relation between the amount of substance 

 absorbed (cv) and its concentration unabsorbed in solution (c) is 

 given by Freundlich's equation : 



- = ^T" 

 m 



where m = weight of adsorbent present, 



A; is a constant depending on the nature of the adsorbent and 

 is the amount adsorbed when c = 1 

 and n is a constant depending on the nature of the adsorbed 

 substance. 



(The value of n is usually about 0-5.) 

 Four further points are of interest : 



(1) The rapidity with which adsorption takes place. 



(2) The reversible nature of adsorptions of the purely physical type. 

 If definite amounts of adsorbent and adsorbable substances are 

 allowed to attain equilibrium and then the concentration of either 

 of them altered, a new equilibrium point will be reached. For 

 example, if after charcoal has removed a certain quantity of a 

 solute from a solution, the solution be diluted with an equal 

 quantity of water, so reducing the concentration of the charcoal 

 and of the unabsorbed material by half, then some of the material 

 will be given up by the charcoal to the solution. The final con- 

 centrations will be the same as if one had started with half the 

 quantity of charcoal. 



(3) The phenomenon of adsorption-displacement. If two or more 

 adsorbable substances are present in solution and no chemical action 

 takes place only one substance will be adsorbed — the more sparingly 

 adsorbable substances will remain in solution. On the other hand, 

 if a substance A is adsorbed and then B which is more powerfully 

 adsorbed is added to the solution, A will be completely expelled 

 from the surface. Capillary active substances, by powerfully 

 lowering surface tension, cause the expulsion of all less active 

 substances from a charcoal- water interface. (See Chap. XI., 

 Adsorption Membranes, and Part II., Experiments 18 and 20.) 



