ACTION OF ANTITOXINS ON TOXINS. 265 



(toxin and antitoxin) combine in variable proportions. We may 

 imagine that each molecule of toxin is able to unite with or fix a 

 variable number of molecules of the antitoxin. Let us suppose, for 

 example, that a molecule T of toxin can unite either with a single 

 molecule or with 2, 3, 4 or 5 molecules A of antitoxin. Five 

 compounds then are possible that may be designated TA 1 , TA 2 , 

 TA 3 , TA 4 and TA 5 . These mixtures will be more or less toxic, 

 according to the amount of antitoxin present. The first, TA 1 would 

 be rather poisonous, although less so than pure toxin; the following 

 TA 2 and TA 3 would be successively less toxic; TA 4 and TA 5 may 

 be supposed to have no toxic effect. 



If we were to mix with one volume of toxin containing 100 mole- 

 cules (T) a volume of antitoxin containing 200 molecules (A), we 

 should have a compound TA 2 . The antitoxin would be equally 

 distributed over all the molecules present to form a compound of 

 distinct though slight toxicity. This compound represents the 

 toxic molecule partially saturated with antitoxic molecules. It is 

 an attenuated but not a neutralized toxin. 



If to the same number of toxin molecules (100 T) we were to add 

 300 molecules of antitoxin, the compound TA 3 would be formed, 

 and so on. In each instance, and notwithstanding the dosage, the 

 antitoxin will be equally distributed among all the toxic molecules. 

 The resultant compounds would differ according to the amount 

 of antitoxin. As a result, we would never find in such mixtures 

 quite free and intact toxin in conjunction with a toxin completely 

 saturated with antitoxin. 



On the other hand, if toxin and antitoxin united regularly in a 

 fixed, constant and uniform proportion, it would be easy to obtain 

 a mixture containing both intact toxin and saturated toxin by 

 adding to a certain volume of toxin a relatively small amount of 

 antitoxin. 



In brief, we must regard a mixture of toxin with an incomplete 

 dose of antitoxin* as one or the other of two very different com- 

 pounds, in accordance with whether we accept the hypothesis of a 



* We may note at once that it is precisely such non-fatal mixtures that give 

 rise to Ehrlich's phenomenon, that is, mixtures containing a dose of antitoxin 

 capable of neutralizing 100 lethal doses of toxin plus a slight excess of toxin, say 

 120 doses. 



