398 BACTERIOLOGY. 



this dilution of the test toxin contains one unit of anti- 

 toxin. In the French method the amount of antitoxin 

 which is required to protect a mouse from a dose of 

 toxin sufficient to kill in four days is determined, and 

 the strength of the antitoxin is stated by determining 

 the amount of serum required to protect one gramme 

 of animal. If 0.001 c.c. protected a 10 gramme mouse 

 the strength of that serum would be 1 : 10,000. Guinea- 

 pigs are sometimes used in place of mice. Knorr's toxin 

 is preserved by precipitating it with saturated ammo- 

 nium sulphate and drying and preserving the precipi- 

 tate in sealed tubes. As required, it is dissolved in 10 

 per cent, salt solution, as above stated. For small 

 testing stations the best way is to obtain some freshly 

 standardized antitoxin and compare serums with this. 



The Persistence of Antitoxin in the Blood. Ransom 

 has recently shown that the tetanus antitoxin is elimi- 

 nated just about as rapidly from the blood of an animal 

 when produced by toxin injections as when injected 

 with antitoxin, so long as the serum was from an ani- 

 mal of the same race. When from a different race, it 

 is much more quickly eliminated. From this we see 

 a possible explanation of the fact that immunity in 

 man, due to an injection of the antitoxic serum of the 

 horse, is less persistent than immunity conferred by an 

 attack of the disease. 



He found some interesting facts in testing the anti- 

 toxic values of the serum of an immunized mare, of its 

 foal, and of the milk. The foal's serum was one-third 

 the strength of the mare's, and one hundred and fifty 

 times that of the mare's milk. In two months the 

 mare's serum lost two-thirds in antitoxic strength, the 

 foal's five-sixths, and the milk one-half. Injections of 



