DIPHTHERIA BACILLUS. 373 



tested antitoxin is of immense value in insuring a 

 uniform, though not necessarily correct, standard among 

 the different testing stations and in allowing of com- 

 parison between them. 



The old definition of Behring and Ehrlich, that an 

 antitoxin unit contains the amount of antitoxin which 

 will protect the life of a guinea-pig from one hundred 

 fatal doses of toxin, must be modified so as to be de- 

 fined as that amount of antitoxin which will neutral- 

 ize one hundred fatal doses of a toxin similar to that 

 adopted as the standard namely, one having the char- 

 acteristics of toxins in cultures at the height of their 

 toxicity. 



The actual test of an antitoxin serum is, therefore, 

 carried out as follows: Six guinea-pigs are injected 

 with mixtures of toxin and antitoxin. In each of the 

 mixtures there is 100 times the amount of a toxin such 

 as just described, which will kill 250 grammes of 

 guinea-pig on an average in 96 hours. In each of the 

 mixtures the amount of antitoxin varies; for instance, 

 No. 1 would contain 0.002 c.c. serum, No. 2, 0.003 

 c.c., No. 3, 0.004 c.c., No. 4, 0.005 c.c., etc. If, at 

 the end of the fourth day, Nos. 1, 2, and 3 were dead, 

 and Nos. 4, 5, and 6 were alive, we would consider 

 the serum to contain 200 units of antitoxin for each c.c. 

 When we mix only ten fatal doses of toxin with one- 

 tenth of the amount of antitoxin used with 100 fatal 

 doses we usually consider that the guinea-pig must not 

 only live but remain well. 



The Relation of Bacteriology to Diagnosis. I believe 

 that all experienced clinicians will agree that, when 

 left to judge solely by the appearance and symptoms 

 of a case, there are certain mild exudative inflamma- 



