266 STl'DIKS IN IMMUNITY. 



combination in fixed proportions, or the hypothesis of union in 

 variable proportions. In the first instance we conceive of the 

 fluid as containing two substances — free active toxin and saturated 

 and inactive toxin. On the second supposition we conceive of the 

 mixture as containing a single substance — an incompletely satu- 

 rated, non-neutralized, or simply an attenuated, toxin. 



It is evident that non-identity in the composition of the mixtures 

 would lead to a marked ilijference in action on the animal body. And, 

 in accordance with the view adopted as to the union of the antago- 

 nistic substance, one would presuppose very different harmful 

 properties in a given fluid. It is quite understandable, however, 

 that a fluid containing attenuated toxin should be less dangerous 

 than one containing a certain dose of intact toxin along with com- 

 pletely neutralized toxin. Ehrlich's phenomenon would appear 

 easily explicable if we accept the idea of a combination in variable 

 proportions. 



The preceding statement is evidently schematic, and it is the 

 fault of such schemes to be, in general, too dogmatic. Nor is our own 

 working hypothesis free from this criticism, and should therefore not 

 be taken too literally. For example, it would imply that the union 

 of toxin and antitoxin is a simple molecular union; it also of neces- 

 sity implies that our two substances obey the law of multiple pro- 

 portions strictly (as that the dose of antitoxin in TA 3 for example, 

 is an exact multiple of that in TA). As a result, therefore, in order 

 to simplify expression we may look on each substance in question 

 as one elemental particle, as one molecule, which is by definition 

 indivisible. It is evident that our purpose is not to determine 

 whether toxin and antitoxin unite by a union of molecules or by 

 an exchange of atoms, nor to find out whether we are dealing with 

 combinations in exactly multiple, or simply in variable, proportions. 

 Such problems are beyond our present range, and experiment is 

 unable to solve them. 



We are dealing with a single idea, and our schema has been used 

 simply to express this idea. If the hypothesis of a union in variable 

 proportions is exact, the essential characters of the reaction would 

 be as follows: 



1. When, to a given amount of toxin, antitoxin is added in an 

 amount that does not suffice completely to neutralize, the anti- 



