64 PRINCIPLES OF GENERAL PHYSIOLOGY 



Arrhenius applies only to u very limited number of cases of adsorption, so that it is probable 

 that the fact of the satisfactory application of this theory to certain cases is due to the 

 connection of surface tension with molecular attraction, in arrnnlunce with the Yoaog-Laplaoe 

 theory. In any case, as will be clear from what is said in other parts of the present chapu-i , 

 we must admit that the actual process of adsorption in any particular case is a complex of 

 several factors. 



The taking up of arsenious acid by ferric hydroxide introduces us to the study 

 of an important class of substances called " adsorption compounds" or, by some, 

 "colloidal complexes." Although we are still dealing with surface action, the 

 surfaces in question are those of the minute particles of matter in the colloidal 

 state, and the complexes formed behave in many ways like true chemical com- 

 pounds. How the two are distinguished will be shown in the following section. 



ADSORPTION COMPOUNDS 



If we take a (colloidal) solution of the free acid of Congo-red, which has a blue 

 colour, and add to it, quickly, a solution (also colloidal) of thorium hydroxide, a 

 precipitate of a blue colour is formed. This precipitate can be filtered off, or 

 better, centrifuged off, and resuspended in water. On allowing it to stand at room 

 temperature, it slowly becomes red and part of it goes into solution : this change 

 can be produced quickly by boiling. What is the explanation of this phenomenon? 



The surfaces of the particles of the Congo-red acid have a negative charge, as 

 can easily be shown by the behaviour to charged electrodes. The particles of the 

 thorium hydroxide, on the other hand, have a positive charge. By aggregation 

 together of these two substances the charges neutralise one another and free 

 energy disappears, so that such a process will occur. But chemical combination 

 only takes place very slowly, owing probably to very slight degree of ionisation of 

 these two colloids. We have, in fact, free acid and free base in close apposition, 

 but uncombined, as shown by the blue colour, which is that of the free acid 

 When chemically combined, the salts have a red colour, such as appears on heating 

 the adsorption compound, or slowly at ordinary temperature. There are certain 

 precautions to be observed to ensure success in this experiment, for which the 

 reader is referred to my paper (.1911, i. p. 83). 



This peculiar type of compound is commonly met with where colloidal bodies 

 are present, as in living organisms. It is rarely, however, that the nature of the 

 complex is as clear as in the case given. Other properties must usually be taken 

 into consideration. One of these is the absence of any quantitative, stoichiometric, 

 relation between the constituents of the compound ; they may be present in any 

 ratio whatever. The colloidal complex of ferric chloride and ferric hydroxide, 

 present in dialysed solutions of ferric chloride, may contain any percentage of 

 chlorine from 65'5 (that of the chloride itself) through all stages to 6'4 per cent. 



It will perhaps assist the reader to realise the distinction between chemical 

 combination and adsorption if a few actual cases are considered briefly. 



When a given quantity of charcoal is in equilibrium with solutions of acetic 

 acid of varying concentrations, for each concentration there is a definite amount 

 present in both phases, that is, there is always more or less acetic acid left in 

 solution, however small the amount originally present. In seeking for a true 

 chemical reaction to compare with this, it must be remembered that the acetic 

 acid adsorbed on the surface of the charcoal is, for the time, fixed there ; it is not 

 in solution. The adsorption compound is similar to a precipitate. Our chemical 

 reaction must therefore result in the production of a precipitate. Take, then, silver 

 nitrate, and add to it varying percentages of sodium chloride. What happens i> 

 familiar to every one. At all concentrations of sodium chloride less than that 

 equimolar with the silver present, all the chlorine is carried down and none is left 

 in solution ; at all concentrations of sodium chloride greater than equimolar, the 

 amount of precipitate is always the same, whatever the concentration of the sodium 

 chloride. The graph, instead of being parabolic, like that of adsorption, consists 

 of two straight lines at right angles to one another. The figure by Freundlich 

 (1909, p. 287) shows this in the case of the combination between diphenylamine 

 and picric acid as investigated by Appleyard and Walker (Journ. Chem. i'oc., 69, 



