IMMUNITY, IMMUNIZATION, AND CURE. 65 



somewhat inconvenient. The characteristic islands are then 

 seen to form at the periphery of the drop. 



Gruber's reaction can also be obtained with the serum of 

 normal animals and human beings, but in this instance much 

 more serum is required than when the serum of immune 

 animals is employed. In the case of normal serum the pro- 

 portion in the large majority of cases exceeds i : 10 that is, 

 the addition of more than one drop of serum to ten drops 

 of bouillon is required to induce agglutination. Only in 

 rare instances has a proportion of I : 30, or even of I : 40, 

 been noted in human beings. The serum of rabbits, horses, 

 asses, possesses agglutinating activity that varies between 

 I : 30 and I : 40. The serum of guinea-pigs, as a rule, ex- 

 hibits no agglutinating property whatever. It is, therefore, 

 absolutely necessary in studying the agglutinating reaction 

 to consider the quantitative relations in every instance. In 

 applying the test according to either the first or the second 

 of the methods described, several observations are always 

 made. To the bouillon is first added serum in the propor- 

 tion of I : 10, in a second test-tube in the proportion of I : 20, 

 then i : 30, I : 50, I : 100, etc. 



With an observance of the precautionary measures noted 

 the reaction of Gruber is of great service in the differential 

 diagnosis of bacteria that resemble one another morpho- 

 logically. It is preferable to the reaction of Pfeiffer, as it is 

 much more easily performed. To identify cholera-bacilli, 

 typhoid-bacilli, etc., the serum of animals immune to 

 cholera, typhoid fever, etc., is tested to determine its power 

 of causing agglutination of bouillon-cultures of the respect- 

 ive organisms. If the reaction is negative when a dilution 

 of i : 10 is employed, it may be concluded with certainty 

 that the bacteria under examination are not identical with 

 those to which the animal yielding the serum has been 

 rendered immune. If, however, the reaction prove posi- 

 tive, the result is in favor of the conclusion that the micro- 

 organisms are the same. To establish completely the 

 identity of the microorganism it is necessary, however, to 

 determine how far the degree of dilution can be carried. In 

 individual instances this may reach several thousand. The 

 proportion of from i : 50 to i : 75 is, however, quite suffi- 

 cient, and such a dilution will be satisfactory if but a small 

 amount of serum is available. The extreme limit of agglu- 

 tinating activity is best determined with the aid of the 

 5 



