322 ADSORPTION 



tion between two substances dependent on physical attraction 

 rather than on chemical affinity, and taking place in very variable 

 ratios, rather than in simple ones dependent on combinations of 

 atoms or molecules, such as occurs in true chemical union. But 

 it must be admitted that there is no absolutely sharp line of 

 demarcation between the two classes of phenomena. In most 

 cases the two substances entering into the phenomenon of adsorp- 

 tion exist as such side by side in the compound, which is to be 

 regarded rather as an intimate admixture of the two than as an 

 entirely new substance. The question arises as to whether the 

 union between antigen and antibody is not really a process of 

 adsorption, and in the attempt to solve this problem several very 

 startling analogies with colloidal adsorption have been discovered. 

 It will be convenient to discuss some of these, and then to give 

 an account of the analogies met with in the chemistry of the 

 colloids. 



Bordet has shown that the amount of haemolytic immune body 

 which can be taken up by a given volume of corpuscles varies 

 according to whether the corpuscles be added at the same time or 

 in successive small portions. Thus, in one example 0*4 c.c. of a 

 haemolytic serum dissolved 0-5 c.c. of corpuscles if added at once. 

 But if 0-2 c.c. of corpuscles were added first, and then successive 

 amounts of o-i c.c. put in, no solution took place after that of the 

 first portion added. This he explains, as we have already seen, by 

 invoking a physical process of the nature of adsorption, comparing 

 it with the adsorption of a dye by filter-paper. Other explanations 

 may be possible, but an exactly similar phenomenon may be seen 

 in the mutual adsorption of colloids of opposite sign. Thus, if to 

 a given amount of solution of an electro-positive colloid an amount 

 of solution of an electro-negative colloid exactly sufficient for 

 neutralization of the opposite charges be added, the result will be 

 the immediate commencement of the process of agglutination, 

 which will go on until all the colloids are precipitated. But 

 if a small amount of the second colloid be added to the same 

 volume of the other, a different state of affairs is brought about- 

 New aggregates of the two are formed, which are not necessarily 

 neutral in reaction, and experience shows that the conditions are 

 now less favourable to precipitation, which may require much 

 more of the second colloid for its complete accomplishment, the 

 intermediate bodies being apparently less easily precipitated than 

 the unaltered colloid. Similar phenomena may also be seen in the 



