IX 



yellow colour and B. coli a red colour, the original 

 colour of the medium, due to differences in reaction, 

 being quite immaterial to the final results. 



Although this method possesses the following 

 advantages : 



(a) The results, in comparison to those of the older 

 method, are dependent, to a minor degree only, 

 upon the quantity of free acid or alkali present 

 in the medium. 



(b) When once the typical colours have been obtain- 

 ed, no further alteration lakes place. 



(c) It is possible to differentiate certain members 

 of the typhoid and coli groups which could not 

 be diagnosed by the former method. 



it must be remembered that: 



(a) Possibly some strains of B. typhosus and B. coli 

 will not produce the colour change. 



(b) Different stocks of neutral red do not seem to 

 be equally reducible. 



38 



