138 ESSAYS ON BACTERIOLOGY. 



The bacteria generally believed to be the cause of 

 typhoid fever are so constantly associated in the ex- 

 cretions mth others resembling them that they can- 

 not be identified by microscopic examination nor by 

 ordinary methods of cultivation. Bacteriology there- 

 fore does not, unfortunately, offer to the physician 

 any direct means of diagnosis in this disease. But of 

 late a procedure has come forward illustrating the in- 

 direct application of bacteriological diagnosis. It has 

 been observed that if typhoid bacilli are brought into 

 contact with the blood, serum of one suffering from 

 typhoid fever, they lose their motility and gather into 

 swarms. This Widal test is applied as follows: A 

 pure culture of typhoid bacilli being at hand, and 

 having been examined to make certain that the ba- 

 cilli show the normal motility, a particle of the cul- 

 ture is thoroughly mixed with distilled water, or, still 

 better, a fresh culture is made in bouillon, and from 

 this a few drops are taken. A drop of fr^h blood or 

 of dried blood moistened with distilled water is now 

 added. From this mixture of typhoid bacilli and sus- 

 pected blood a portion is transferred to a cover glaas, 

 the latter being immediately inverted over a concavity 

 in a glass slide. The microscope now soon shows the 

 bacilli gathering into swarins or clumps, and at the 

 same time losing their motility. Should both of 

 these phenomena occur, the reaction is said to be typ- 

 ical, and the inference is made that the blood came 

 from one suffering from typhoid fever. The time re- 



