222 CLINICAL BACTERIOLOGY. 



for 25,000 grams in weight of guinea-pig. As may be 

 readily seen, this mode of calculation agrees entirely with 

 the first described. According to that, o. I cu. cm. of 

 serum was capable of neutralizing 10 times the minimal 

 lethal dose ; while according to this, I cu. cm. of serum is 

 capable of neutralizing 100 times the minimal lethal dose in 

 the typical guinea-pig weighing 250 grams. 



Roux estimates the immunizing power in quite another 

 manner. He determines how much serum must be injected 

 from twelve to twenty-four hours previously, in order to 

 protect a guinea-pig against the lethal dose of diphtheria- 

 bacilli or of diphtheria-toxin that otherwise would have de- 

 stroyed the animal in not more than thirty hours. He fixes 

 as the power of the serum that figure which expresses the 

 relation between the amount of serum and the body -weight 

 of the guinea-pig. An immunizing power of 20,000 in- 

 dicates thus that of this serum Yolroo" P art f ^ e body- 

 weight of a guinea-pig must be injected in order to protect 

 the animal against the amount of diphtheria-culture or 

 diphtheria-toxin that would cause death in thirty hours. 



Spronck has proposed a special formula for converting 

 the immunity-unit of Roux into that of Behring. B -^ 

 that is, it is only necessary to divide Roux's figures by 

 500 in order to obtain the number of Behring units. 



In a communication upon the estimation of the strength 

 of diphtheria-antitoxin and its theoretic basis Ehrlich starts 

 with a dry diphtheria-antitoxin prepared in the Hochst 

 works in order to obtain a sufficiently constant standard and 

 a unit of measure for the serum that is protected from the 

 destructive influence of water, oxygen, light, and heat. 

 This contains 1 700 immunization-units or normal antitoxin- 

 units to the gram. It is preserved in a small apparatus con- 

 sisting of two communicating glass tubes, of which one con- 

 tains the powder and the other phosphoric anhydrid as a 

 dehydrating agent. The apparatus is most carefully ex- 

 hausted of air, and kept in a dark, cool place. Each tube 

 contains 2 grams of antitoxin-powder, which before being 

 used is dissolved in 200 cu. cm. of a solution of sodium 

 chlorid (10 per cent.) and glycerin (from 50 to 80 per 

 cent). One cubic centimeter of this solution diluted 17 

 times represents the immunity-unit or normal antitoxin-unit. 

 If to this immunity-unit are added increasing quantities of 

 diphtheria-toxin, it is possible to fix two limits (L), which 



