110 BACTERIOLOGY 



is indicated by the appearance of a pink color, the effect of 

 the alkali on the phenolphtalein. Froni the amount of one- 

 twentieth normal alkali solution needed for 5 c.c. of the 

 medium it is easy to calculate the number of cubic centi- 

 meters of the normal solution that will be required to neu- 

 tralize the entire mass. 



The phenolphthalein neutral point lies so high, averaging 

 47 c.c. of normal caustic alkali solution per liter for nutrient 

 meat-infusion agar-agar, and 56 c.c. per liter for nutrient 

 gelatin, that it is improbable from experience gained from 

 the older methods that the conditions offered by media 

 neutral to this indicator are suitable for the growth of all 

 bacteria, so that with particular species it may be neces- 

 sary to determine by experiment the degree of deviation 

 from the neutral point that is best suited for development. 

 In Fuller's experience the degree of deviation from the 

 phenolphthalein neutral point that gives in general the best 

 results is represented by from 15 to 20 of his scale i. e., 

 there should remain enough uncombined acid in a liter of 

 the finished medium to require the further addition of caustic 

 alakali to the extent of from 15 to 20 c.c. of a normal solution 

 to bring the reaction of the mass up to the phenolphtalein 

 neutral point. Thus, for example, if upon titration it should 

 be found that to neutralize a liter of nutrient meat-infusion 

 gelatin by the phenolphtalein process 55 c.c. of normal 

 caustic alkali solution would be needed, the amount actually 

 added would be from 35 to 40 c.c. i. e., from 15 to 20 c.c. 

 less than the amount needed to bring the reaction up to the 

 neutral point. 



Not infrequently the filtered bouillon, neutralized and 

 sterilized, will be seen to contain a fine, flocculent precipi- 



