H4 PHYSICAL CHEMISTRY IN MEDICINE. 



foundation of our modern conceptions of its nature. If 

 we add to a certain amount of a colloid A a much smaller 

 amount of a neutralizing colloid B, then, generally 

 speaking,* a part of A is not completely neutralized, while 

 the remaining part remains free, but B is distributed 

 over and combines with as large a number of the colloidal 

 particles of A as is possible. As a result, new, incom- 

 pletely neutralized aggregates are formed. By this means 

 the number, size, and electrical condition of the particles 

 become changed, and it depends entirely upon the char- 

 acter of these new values whether the tendency toward 

 a formation of further aggregates upon the addition of 

 a second portion of B is favored or inhibited. When 

 such a second addition is made, new aggregates with 

 new properties are again produced. If we have deter- 

 mined the amount of antitoxin necessary to just neu- 

 tralize a lethal dose of toxin, we will need n times this 

 amount to neutralize an w-times dose of toxin. Things 

 are different, however, as soon as we try to saturate 

 gradually, through the addition of succeeding small 

 amounts of antitoxin, an n dose of toxin, as EHRLICH 

 has done. Under these circumstances we can get only 



* It cannot be discussed in this paper in how far neutralization 

 velocity and reversibility of the formation of aggregates determine the 

 character of the course of the reaction. The more important instances 

 can, however, be reviewed. Reversibility is always only partial, and 

 decreases steadily from the moment of neutralization. Even such 

 stabile colloidal changes as the coagulation of egg albumin through 

 heat or concentrated mineral acids are reversible at the moment that 

 they are brought about. In the precipitation of protein through phenol, 

 alcohol, or neutral salts the effect of time and the degree of reversibility 

 increase from the first toward the last. This general property of 

 colloids has in recent discussions of immunity assumed an important 

 part under the heading of secondary fixation of toxin -antitoxin. 



