216 INFECTION AND IMMUNITY. 



slide. More accurate dilutions can be made by 

 means of capillary tubes. 



A convenient amount of serum is allowed to 

 enter the tube by capillary attraction. The length 

 of this column is marked on the tube and then 

 successive volumes of the diluent drawn in, each 

 being separated from the succeeding one by a small 

 bubble of air. It is readily seen how with even a 

 minute quantity of serum, one may make the test 

 with dilutions of one in ten, one in twenty, one in 

 thirty, one in forty, etc., details which are neces- 

 sary for a correctly performed test. It is im- 

 portant that in the different dilutions the same 

 amount of bacterial emulsion be used. 



In the macroscopic test, more serum is neces- 

 sary, though the quantity need not be large, and 

 the dilutions are made in test tubes of suitable 

 size. One should always deal with definite quan- 

 tities of the serum dilutions, and should always 

 add the same amount of bacterial emulsion in the 

 various tubes involved in a test. 



The Micro- If agglutination occurs in the microscopic prep- 

 Reaction? aration described above, one sees, with the high 

 power, in the course of from fifteen minutes to a 

 half-hour, that two or more micro-organisms 

 which come in contact have a tendency to remain 

 in this position. In the case of a motile organism 

 (typhoid) the movements may be exaggerated for 

 a time. In the course of the next few hours, other 

 cells are added to incipient groups and new groups 

 originate. Motility becomes less and less and event- 

 ually ceases, in a characteristic reaction. The 

 maximum change has taken place in from six to 

 eight hours. Not less than four or five cells which 



