BACTERIOLYSIS AND ALLIED PHENOMENA 189 



amount of the emulsion added, each tube made up to a definite 

 volume, and all incubated for one to four hours. At the end of 

 this period a uniform quantity is withdrawn from each, and 

 plates prepared either by mixing with melted agar, or gelatin 

 where suitable, or by smearing over ready-poured agar plates. 

 The amount must, of course, be the same in each case, and may 

 be easily withdrawn by means of one of Wright's pipettes, which 

 is sterilized after use by being washed out several times with 

 boiling water or oil at 150 C. The plates are then incubated, and 

 the colonies which develop after twenty-four or forty-eight hours 

 are enumerated, and the amount of serum which kills all or the 

 greatest number of bacteria is noted. 



Certain controls are necessary, the main being (a) a tube 

 inoculated as above, but without the addition of serum ; and 

 (b) a tube also containing bacterial emulsion, and also a relatively 

 large amount of heated serum. The main error comes in from 

 the reduction of the number of colonies in consequence of aggluti- 

 nation, but this can be discounted in some measure by comparison 

 with the plate prepared from control (&). 



Other methods are employed, notably that of Wright, for which 

 the original article should be consulted. The value of the pro- 

 cesses is not great, since it does not tell us even the actual 

 bactericidal value of the circulating blood (since we do not know 

 the amount of complement which is available) nor the amount of 

 immune body. In some cases a serum containing a large amount 

 of the latter substance will show little or no bactericidal power in 

 vitro, owing to the deficiency in complement, and may require the 

 addition of a hundred times its volume of normal serum to be 

 fully complemented. 



To determine the relative amount of immune body present, the 

 principle of the method used for the measurement of the haemolytic 

 amboceptor is adopted, a series of mixtures of constant amounts 

 of bacterial emulsion and fresh normal serum is prepared, and 

 varying amounts of the heated immune serum to be tested are 

 added, the whole made up to uniform volume, and treated as 

 above. Here a further control is necessary, since the fresh 

 normal serum may contain some immune body or be otherwise 

 bactericidal. One of Neisser and Wechsberg's examples of this 

 process has been already quoted. 



The determination of the amount of bactericidal complement is 

 simple enough theoretically, and follows the same lines as that 



