THEORIES OF IMMUNITY. 



experiments with porcelain filters coated with 

 gelatine were also brought forward to show 

 that the reaction is an actual chemical combina- 

 tion. In them it was found that at first, in mix- 

 tures of toxine and antitoxine, the latter would 

 not pass through, but the former would; later, 

 however, a neutral fluid would appear, which was 

 interpreted as showing that an actual chemical 

 combination had taken place. 



Subsequent observations showed that a simple 

 chemical reaction was not .sufficient to explain 

 all the conditions. It was found that toxines 

 lost a considerable degree, if not all of their 

 power, if allowed to stand for some time, an ob- 

 servation that had been made by many observers ; 

 but Ehrlich also found that this weakened toxine 

 — to which he gave the name " toxoid " — re- 

 quired the same amount of antitoxine to neutral- 

 ize it as when fresh. In other words, the toxic 

 power of the toxine had diminished, whilst its 

 combining power remained the same. This being 

 true, the reaction could not be considered the 

 same as a simple chemical neutralization. 



46 



