COLLOIDS 69 



against concentration of reacting substances) is a parabola, indicating 

 that the laws of mass action (page 23) are no longer followed. In 

 order to be able to determine whether some particular process as, for 

 example, a fermentation process, or the absorption of oxygen by blood 

 is caused by adsorption, we must compare its curves, constructed ac- 

 cording to the same principles, with the typical adsorption curve. A 

 formula may be used in constructing the curves. In arriving at this 

 formula, two facts have to be remembered: (1) As adsorption proceeds 

 and less and less of the free energy on the adsorbing surface remains 

 to be neutralized, the reaction slows off, until equilibrium is reached. 

 The more dilute the solution, the greater is the proportion of its con- 

 tents to be adsorbed, which means that if a is the amount of substance 

 adsorbed from a certain solution, then, from a solution of twice that 

 strength, somewhat less than 2 a will be adsorbed i. e., a multiplied 

 by some root of 2. Although the formula is one belonging to the class 

 known as parabolic, it must not be assumed that every reaction which 

 happens to give such a parabolic curve (such as the combination of 0., 

 with hemoglobin under certain conditions) (see page 383) must be one 

 dependent on adsorption. 



It must be understood that although the substance that is removed 

 from a solution by adsorption is no longer capable of contributing to the 

 conductivity or the osmotic pressure of the solution, it is nevertheless 

 not so firmly fixed that it can not be set free again by purely mechanical 

 means, as by constant dilution of the fluid. If charcoal which has ad- 

 sorbed sugar is placed in a dialyzer made of membrane the pores of 

 which allow sugar but not charcoal to pass through, the sugar will 

 gradually be removed if the dialyzer is immersed in running water. A 

 certain equilibrium exists between the substance adsorbed and the same 

 substance still remaining in solution. If the latter is constantly dimin- 

 ishing by dialysis, the adsorption compound must break down to main- 

 tain the equilibrium. It is clear, however, that the process of removal 

 will be extremely slow. The ability of adsorbed substances to withstand 

 removal by washing is taken advantage of by nature in holding back 

 foodstuffs in the soil. 



Physiologic Processes Depending on Adsorption 



Instances in which adsorption undoubtedly plays a most important 

 part in physiologic processes are as follows: 



1. The action of enzymes (see page 71). 



2. The combination of toxin with antitoxin occurs according to the laws 

 of adsorption rather than those of mass action. In this case it is im- 

 portant to note that when the toxin of diphtheria is added in small sue- 



