82 



Dr. W. M. Bayliss. 



[Apr. 1, 



The mathematical expression correlating the concentration of the body in 

 solution with the amount adsorbed by a particular surface in contact with 

 this solution is of an exponential form, which is undoubtedly due to the 

 manner in which the degree of diminution of surface energy is related to the 

 amount adsorbed. We may say, then, that adsorption is, as a rule, the 

 second stage, in a heterogeneous reaction. 



If the bodies brought into close contact by the above-mentioned surface 

 condensation do not react with one another in the chemical sense, the whole 

 process ends at this stage. A case of this kind is that of the condensation of 

 aniline on the surface of mercury, investigated by Lewis.* Complete absence 

 of chemical reaction is, however, not common. Where it occurs the rate at 

 which it takes place varies considerably in different cases. It is important 

 to remember, indeed, that, in agreement with the law of mass action, this 

 velocity will be a function of the amount adsorbed, and therefore much 

 greater than if no surface condensation had taken place. Chemical reaction 

 is the third and last stage of heterogeneous reactions, and is, as a rule, the 

 stage which conditions the rate of the process as a whole. Adsorption is 

 rapid, and diffusion has not usually to take place through more than very 

 short distances.^ 



When colloidal solutions are concerned, the " disperse " phase may consist 

 either of ultra-microscopic particles of a solid, or of droplets of a liquid 

 immiscible with that in which they are suspended. Moreover, the body 

 adsorbed may be either in true solution or colloidal solution. 



It will be obvious that, even if the adsorbed body does not enter into 

 chemical combination with that upon whose surface it is condensed, never- 

 theless a kind of complex is produced, which may be separated from the rest 

 of the system. Such bodies have been called " adsorption compounds," or, 

 when both components are colloids, " colloidal complexes." In most cases it 

 is a matter of some difficulty to show what their real nature is ; their 

 existence even is denied by many investigators. Assuming that such 

 compounds are formed in any particular case, the velocity of the subsequent 

 chemical reaction will be a function of the amount of the adsorption 

 compound in existence at a given instant of time, and this again is an 

 exponential function of the concentration of the solution of the adsorbed 

 body. Accordingly, the form of the expression correlating the velocity of 

 the reaction with the concentration of the reagents will also be an 

 exponential one. 



* ' Zeit. f. physik. Chem.,' 1910, vol. 79, pp. 129—147. 



t See Arrhenius, ' Immunochemistry,' p. 142, New York, 1907. Also Loevenherz, 

 4 Zeit. f. physik. Chem.,' 1894, vol. 15, pp. 389—398. 



