282 A MANUAL OF BACTERIOLOGY 



because the first portion of the added toxin simply dis- 

 places the toxone from its combination with the antitoxin, 

 and is neutralised by the antitoxin so set free. 



Thus, suppose a certain amount of a toxin broth contains 

 90 units of toxin and 10 units of toxone, and to this amount 

 100 units of antitoxin are added so as to form a physiolo- 

 gically neutral mixture, the combination which occurs is 

 shown by the following " equation " : 90 toxin-antitoxin -f- 

 10 toxone-antitoxin = L (i.e. neutrality). If an amount 

 of the toxin broth be now added, corresponding to 11 units 

 of toxin, the effect will be as though only one unit of toxin 

 has been added, as is shown by the following " equation " : 

 90 toxin-antitoxin + 10 toxone-antitoxin + 11 toxin = 

 100 toxin-antitoxin + 10 toxone (free) + 1 toxin (free) = 

 L + (i.e. just acutely lethal). Thus although the equivalent 

 of eleven minimal lethal doses of toxin has been added to 

 the physiologically neutral mixture of toxin broth and 

 antitoxin, only one minimal lethal dose of toxin remains 

 free and active, because ten toxin units displace the ten 

 toxone units from the toxone-antitoxin complex and are 

 neutralised by the antitoxin thus set free. Ehrlich, there- 

 fore, devised a method of standardisation which eliminates 

 irregularities due to the variable proportions of toxone 

 and toxin in the toxin broth by adopting antitoxin and 

 not toxin as the standard. In order to standardise an 

 antitoxin, a virulent toxin broth is employed and its 

 minimal lethal dose is approximately ascertained i.e. 

 that amount which is just sufficient to kill a 250-grm. 

 guinea-pig on the fourth or fifth day. A solution of accu- 

 rately standardised antitoxin, which can be obtained from 

 the Serumspriifung Institut, Frankfort-on-Maine, is then 

 prepared, containing one " unit " of the antitoxin in 1 c.c., 

 and the toxin is standardised with this by mixing with 

 one unit various quantities above and below one hundred 

 minimal lethal doses. It is required to ascertain the 



