vin AGGLUTINATION OF B. PESTIS 211 



being used, and granted also that the sample of blood to 

 be tested is used after the complete separation of the 

 fibrin, the question arises, In what dilution and for what 

 length of time should the test be carried out ? As is well 

 known in the case of an animal well " prepared " with 

 typhoid culture, or with cholera culture, agglutination is 

 positive even when the animal's blood serum is used in 

 very high dilutions — e.g. 1 in 200, 1 in 500, and even 

 1 in 1000 — agglutination (under the microscope) occurring 

 in a decided fashion within the hour. 



In the observations of some authors (Bordet) the 

 addition of normal serum, the placing of the mixture in 

 the hot incubator, and various other factors are introduced 

 which have an accelerating influence on the agglutination. 

 These and similar observations are no doubt, both from a 

 theoretical and practical point of view, of value. But in 

 the case of the agglutination of plague culture I have 

 limited myself preferably to making the test in as simple 

 and uniform a manner as possible, so as to avoid the 

 introduction of quasi - extraneous, for the most part 

 unknown, new factors. The explanation of the process 

 and nature of the phenomenon of agglutination is in itself 

 complex and difficult in its most simple form, and it is 

 made considerably more complex, without adding to the 

 better understanding of it, by introducing a number of 

 unknown additional factors. I have convinced myself 

 early in my experiments that the blood serum of rats 

 previously protected against plague, as also the blood 

 serum of guinea-pigs previously prepared by repeated 

 injection of sub - fatal doses of plague culture, has 

 unmistakably the power to agglutinate the plague bacilli 

 in a salt emulsion of gelatine plague culture when used in 



