196 STRONG AND TEAGUE. 



Two points are strikingly obvious from this series of experi- 

 ments : (1) There is great variability in the limits of agglutina- 

 tion of the different strains and (2) the strains freshly isolated 

 from experimental animals agglutinate only at relatively low 

 dilutions of the serum. It is also to be noted that both pneumonic 

 strains and bubonic ones are agglutinated by the same serum. 



It was next decided to select certain of these strains in order 

 to make a careful study of the influence of various factors upon 

 their agglutinability. 



One of the difficulties in performing agglutination tests, partic- 

 ularly with strains which have been grown on artificial media for 

 long periods of time, has been the tendency of the bacilli to form 

 clumps spontaneously. We found that very small amounts of 

 alkali would prevent this spontaneous flocculation and sedimenta- 

 tion of the suspensions, but on performing tests with such sus- 

 pensions these extremely small amounts of alkali inhibited all 

 agglutination. 



It has already been mentioned that we performed a series of 

 tests in the presence of only a small amount of electrolytes (0.1 

 per cent sodium chloride instead of 0.8 per cent). This served 

 well to render the suspensions homogeneous and durable and 

 agglutination was not inhibited, but there was frequently a 

 disturbing fiocculation with normal serum within very narrow 

 limits, such as is often seen when one colloid is fiocculated by 

 another. As only two or three tubes of a long series were so 

 affected, it was possible to distinguish between this phenomenon 

 and the true specific agglutination. Nevertheless, the method 

 was abandoned and in the tests to be described below both the 

 bacterial suspensions and the dilutions of the serum were pre- 

 pared with 0.5 per cent sodium chloride solution. 



The following tables, selected from a number of similar ones 

 with other strains of plague bacilli, demonstrate clearly the 

 results of our experiments. 



