AGGLUTININS 79 



motility to complete clumping and loss of motility of 

 all. The degree of clumping should be reported as 

 a partial reaction, which, although suggestive, is not 

 necessarily specific for typhoid fever. Various grades 

 of intensity are sometimes seen in the different dilu- 

 tions. A positive reaction with 1 in 40 dilution may 

 practically always be regarded as specific for typhoid 

 fever, though occasionally blood of patients infected 

 with B, paratyphosus A or B may give a positive re- 

 action in this dilution with typhoid bacilli. A positive 

 result with 1 in 80 dilution renders the diagnosis 

 doubly sure. 



B. MACROSCOPIC METHOD OF PERFORMING THE WIDAL 

 REACTION 



For this method small test-tubes, such as are em- 

 ployed in the Wassermann reaction, are used. Serum 

 in different dilutions is placed in the tubes by means of 

 small pipettes, and quantities of bouillon culture of 

 typhoid bacilli are then added. A control tube con- 

 sisting of the culture alone is also used. At the end of 

 12 to 24 hours at 37° C, in the case of a positive re- 

 action, the bacilli become clumped and fall as a sedi- 

 ment to the bottom of the tube, leaving a clear fluid 

 above. In the case of a negative reaction and in the 

 control tube, the fluid remains uniformly cloudy. The 

 reaction can be studied best by means of a reading 

 glass. In this method killed cultures of the typhoid 



