CHEMICAL EQUILIBRIA 121 



method of determining the equivalent proportions 

 is to draw the tangent to the neutralization curve 

 (Fig. 30) at its highest point. The point of inter- 

 section of this tangent with the ;t:-axis gives the 

 quantity of serum equivalent to the quantity of 

 poison used (in this special case 02 76 cc. of anti- 

 toxin are equivalent to 004 cc. of toxin). 



In many cases it is said to have been observed that 

 the first dose of antitoxin exerts no neutralizing action 

 upon the poison examined. From the theory we 

 might conclude that it should exert a greater action 

 than the following equal doses. The diphtheria 

 poison spoken of above, when it was first examined 

 by Madsen in the usual manner, seemed to show the 

 phenomenon that the first parts of antitoxin added 

 did not act as a neutralizer. In this case only those 

 observations were taken into account in which the 

 guinea-pigs examined were killed in three to four 

 days. When I made the recalculation I used all the 

 observations in which the animals died in less than a 

 fortnight and also the observed decrease in weight of 

 the animals. In this way I had a material about ten 

 times greater than that used by Madsen, and then 

 the first admixtures of antitoxin showed themselves 

 to be of a stronger neutralizing action than the follow- 

 ing (see table, p. 1 1 2), when the same quantity of anti- 

 toxin was used. In order to explain the old obser- 

 vations Madsen supposed, as Ehrlich had done 

 before, that the diphtheria toxin contains an in- 

 nocuous substance called prototoxoid, which binds 

 the antitoxin with stronger forces than the toxin itself, 



