1 84 LECTURES ON IMMUNITY 



groups are bound to the central part of the molecule as 

 side chains, known from the chemistry of the benzene 

 derivatives. If now, as is often observed, no other change 

 of the poison takes place than that its toxicity diminishes 

 in a certain ratio, e.g. to 50 per cent, then to explain this 

 peculiarity, Ehrlich must assume that all the partial poisons 

 are weakened to the same degree, which seems very im- 

 probable. From our point of view we might explain the 

 same fact by saying that the constant of equilibrium in 

 the above equation has not been " sensibly " altered by the 

 change in the toxophoric group. This is quite possible, 

 although experience with the constant of equilibrium, the 

 so-called dissociation-constant, in acids teaches us that 

 it is rather sensible to changes in the molecule. But the 

 great distance between the different groups in the poison- 

 ous molecule may have the effect that such an influence 

 is in this case insensible. 1 



There is another possible explanation of the phenomenon. 

 As will be seen in Chapter VIII, many poisons are com- 

 pounds of two different substances. It is possible to sup- 

 pose that even the so-called simple poisons are compounds 

 of two substances, of which the one corresponding to the 

 haptophoric group is bound by antitoxin. If this antitoxin 

 binding property of the poison is relatively stable and 

 present in great excess, and if the poisonous compound is 

 formed of one molecule of each constituent, and is a highly 

 dissociable substance, its quantity will be proportional to 

 the concentrations of the two constituents. Then, evi- 

 dently, the constant of equilibrium would remain unaltered 

 if the group that does not bind antitoxin slowly disappears, 



l Cf. Ostwald: Ztitschr.f. ph. Ch., 3. 374 (] 



