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. 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 blvod 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 
