IX 



respectively orange yellow and yellow, and after the 

 lapse of 3 days the colour of both is yellow and 

 so it remains till at least the 16th day. In several of 

 the other columns similar conditions can be observed, 

 though they are not so well marked, and when the 

 variations in colour are observed through a sufficient 

 number of days, the ultimate shade is the same for B. 

 typhosus and B. coli. 



Table I demonstrates moreover the powerful influence, 

 exerted by the quantity of free acid or alkali, not only 

 on the occurrence of a colour change, but especially on 

 the time of its appearance, and the acid present must 

 be reduced to 0.5 H 2 S0 4 before B. coli is able to alter 

 the colour to yellow within 24 hours. 



That none of the earlier investigators have mentioned 

 or observed the capacity of B. typhosus to reduce 

 neutral red, must be accounted for by their having 

 worked, either with different strains of bacilli or with 

 media not absolutely free from glucose. 



From Table II it is seen that the presence of l / z % 

 glucose lessens, in fact almost prevents the reduc- 

 tion of the neutral red by B. typhosus; at the same 

 time the reducing power of B. coli is considerably 

 diminished. From column 0.5 H 2 SO 4 it will be seen 

 that B. coli is not able to alter the colour at all, and 

 from columns 0.25 H 2 SO 4 and neutral, that it has capa- 

 city for altering the colour from magenta or iron red 

 to yellow within 1 2 days. Table II also shows that 

 the quantity of free acid or alkali present, exerts more 

 influence on the results in a glucose medium, than in one 

 that is sugar free. This is a fact of great importance and 

 since the addition of V 2 % glucose has been recom- 

 mended by former experimenters, without any reference 

 being made to the amount of free acid present in the 



12 



