118 INFECTION AND RESISTANCE 



and the conclusions drawn from them. As a result of such experi- 

 ments Ehrlich concludes : 



I. That the diphtheria bacillus produces primarily two kinds 

 of substances (a) toxin, (b) toxon, both of which bind the antibody. 



II. The toxins (and perhaps also the toxons) may deteriorate 

 and be modified into secondary substances (toxoids) which may be 

 distinguished by their different degrees of affinity for antitoxin. 



III. This classification does not exhaust all possible complica- 

 tions, since each subdivision of toxin consists apparently of equal 

 parts of two different modifications which are similar to each other 

 in their relation to antitoxin but differ in varying resistance to in- 

 fluences of deterioration. A more complete analysis of these condi- 

 tions may be found, together with a series of illustrative spectra, in 

 Ehrlich's article in the Deutsche med. Wochenschr., Sept., 1898, 

 which has been quoted above. 



The complex deductions arrived at by Ehrlich are largely de- 

 pendent, as we have seen, upon strict adherence to the analogy be- 

 tween the toxin-antitoxin reactions and those occurring between 

 strong acids and strong bases. In such cases there is a complete 

 reaction, in which chemical change ceases only when there has been 

 a complete neutralization of one by the other. If, for instance, we 

 mix molecular equivalent amounts of H 2 SO 4 and NaOH, an ap- 

 parently complete change into JSTa 2 SO and H 2 O occurs: 



H 2 S0 4 + 2 NaOH = Na 2 S0 4 + 2 H 2 



The reverse process does not seem to take place, and if traces 

 of uncombined II 2 SO 4 and NaOH are present, as may be theoret- 

 ically assumed, they are so slight in amount that they are not dem- 

 onstrable. There are, however, many chemical reactions in which 

 the process is not a complete one, in that the chemical change does 

 not proceed until the reagents are completely used up. Reaction in 

 these cases ceases when an equilibrium is reached at which there are 

 present definite amounts of the reaction products and of the original 

 substances at the same time. 19 



This occurs when a weak acid is added to a weak base. In such 

 cases the reaction is incomplete and reversible and, together with the 

 neutralization products, both free acid and free base may be present. 

 The conditions are best explained by citing an example of a reversi- 

 ble reaction which is commonly given in text-books of physical chem- 

 istry, namely, the reaction between ethyl-alcohol and acetic acid. 

 (Our citation is taken from Philip's "Physical Chemistry," London, 

 Arnold, 1910) : "When one gram mol. of ethyl alcohol is added to 

 one gram mol. of acetic acid, a reaction takes place which results in 



19 See Cohn. "Vortrage f . Artze iiber Physik. Chem.," Engelman, Leip- 

 zig, 1901. 



