68 PRINCIPLES OF GENERAL PHYSIOLOGY 



The second class of cases is typified by that of a colloidal hydroxide and 

 colloidal acid, as described above. The reacting substances are first brought 

 together by mutual adsorption, and chemical reaction then follows between the 

 whole of the constituents of the system. A very similar case is described by 

 van Bemmelen (1910; p. 486). If barium hydroxide solution be added to colloidal 

 silica, a white precipitate falls, which is found to contain both barium hydroxide 

 and silica, but not in chemical combination. On standing, barium silicate is 

 slowly formed and crystallises. Another case is the action of tannin on leather. 

 According to Freundlich (1909, p. 532), the amount of tannin taken up is con- 

 ditioned by an adsorption process, which is then followed by true chemical 

 reaction, which takes place slowly and results in the formation of insoluble bodies. 



The fact to be insisted upon in these cases where chemical reaction follows 

 adsorption is that the velocity of reaction, as affected by various conditions, does 

 not follow the law of mass action in its usual form. The active mass here is the 

 amount adsorbed on the surface, so that the reaction as a whole will be observed 

 to follow the parabolic law of adsorption. The systems of chief interest to the 

 physiologist are those of which colloids form part. Although these are hetero- 

 geneous systems, the internal or dispersed phase is so minutely divided and evonlv 

 distributed that, in comparison with the cases investigated by Nernst, the rate of 

 diffusion does not appear to play so important a part. We shall have to return to 

 this aspect of the question when treating of enzymes. 



This is perhaps the most appropriate place to refer to some cases of biological 

 interest which illustrate the manner in which adsorption intervenes in a variety of 

 processes. 



1. The power of the soil in holding back soluble salts, so that valuable foods 

 are not washed away by the rain. Shown by the experiment with sand and 

 permanganate solution given by J. J. Thomson (1888, p. 192). 



2. Dr Harriette Chick has shown (1906, p. 247) that the complex organic 

 substances, which are detrimental to the nitrifying organisms in the filter process 

 of sewage treatment, are kept back by adsorption in the upper layers of the filter bed. 



3. The action of certain poisons on micro-organisms has been found to be 

 proportional to the amount deposited on their surfaces (H. Morawitz, 1909, 

 pp. 317-322). 



4. Craw (1905) has shown that the combination between toxin and antitoxin 

 follows more closely as to its laws the phenomena of adsorption than those of 

 chemical combination. Perhaps the most striking fact in this connection is the 

 explanation given of the puzzling phenomenon of Danysz (1902), who found that 

 when a given quantity of diphtheria toxin was added in fractions to antitoxin, 

 more toxin was neutralised than when the same quantity was added at one time. 



To neutralise ricin, the toxic substance from the castor bean, it was found that less 

 antiricin was necessary if addtd to a definite amount in successive quantities than if added all 

 at once. And, if ricin be added in separate doses to a definite amount of antiricin, the same 

 amount of ricin requires more antiricin to neutralise it than if the whole is added at one time. 



This phenomenon also takes place when paper adsorbs Congo red (Bayliss, 1906, 

 p. 222). The explanation is that the same amount of adsorbent will take up 

 relatively more from a dilute solution than from a more concentrated one. 



5. In the taking up of bacilli by leucocytes under the influence of a sensitising 

 fluid (so-called "opsonin"), it was found by Ledingham (1912, p. 359) that the 

 two processes involved both followed the course of an adsorption process. These 

 two parts of the phenomenon are (1) the taking up of " opsonin " by the bacilli, and 

 (2) the ingestion by the leucocytes of the micro-organisms thus "sensitised." 



6. When the toxin of tetanus is introduced into a nerve trunk of a warm- 

 blooded animal, it is carried up to the central nervous system and produces 

 convulsions in. due course. If the same experiment be performed on a frog at 

 8 C. it was found by Morgenroth (1900) that although taken up by the nervous 

 system, no convulsions were produced until the animal was warmed to a tem- 

 perature of about 20 C. This is evidently a similar case to that of the Congo- 

 red acid and thorium hydroxide described above. The toxin, although adsorbed, 

 exerts no action until chemical reaction of some kind takes place on warming. 



