306 MANUAL OF BACTERIOLOGY. 



manifest by the collection of the bacteria into visible masses 

 or flocculi, which form a sediment. Most investigators 

 prefer to watch the results under the microscope, using an 

 ordinary slide, or, better, the hanging-drop. Young cultures 

 less than twenty-four hours old in bouillon, and kept in 

 the incubator, may be used, or better cultures kept at room- 

 temperature for twenty-four hours. Johnston and Mc- 

 Taggart recommend that the bouillon cultures be freshly 

 made each time from stock cultures on agar, which need only 

 occasionally be transplanted. Certain stocks of typhoid 

 bacilli seem especially suited to this reaction, and such a 

 stock should be secured. 



Blood-serum, blister-serum, fresh blood and dried blood 

 have all been tried with success. Blood dried on unglazed 

 paper or cover-glasses as proposed by Wyatt Johnston is ex- 

 tremely convenient. To perform the test, it is mixed with 

 sterilized distilled water, bouillon, or normal salt solution; 

 the objection to it lies in the difficulty of securing an accu- 

 rate dilution. An approximate knowledge of the degree of 

 dilution may be acquired by mixing drops of dried blood 

 of known volume with definite amounts of water, and ob- 

 serving the tints. These should be kept in mind as stand- 

 ards. The dilution may be measured with the hemoglo- 

 1)inometer or with the pipette of the hemacytometer. The 

 Xe\v York Board of Health have found blister-serum satis- 

 factory and easy to obtain. A little of the diluted serum is 

 mixed on the cover-glass with a definite amount of the fresh 

 bouillon-culture, and is examined as a hanging-drop. In 

 a short time the characteristic clumping and loss of motility 

 occur. At the same time a drop of the culture alone, and 

 a drop of the culture mixed with normal scrum-, similarly 

 diluted, should be examined as controls. The dilutions 

 used vary from I part of serum in 30 to I in 50. The 

 higher dilutions are more accurate. The time within which 



