98 BACTERIOLOGY. 



neutral point (slightly on tlie side of alkalinity) is indi- 

 cated by the appearance of a pink color, the effect of 

 the alkali on the phenolphtalein. From 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 centimetres of the normal solution that will be 

 required to neutralize the entire mass. 



The phenolphtalein neutral point lies so high, aver- 

 aging 47 c.c of normal caustic alkali solution per litre 

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

 litre for nutrient gelatin, that it is improbable from ex- 

 perience gained from the older methods that the condi- 

 tions offered by media neutral to this indicator are suit- 

 able for the growth of all bacteria, so that with particular 

 species it may be necessary 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 phenolphtalein 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 litre of the finished me- 

 dium to require the further addition of caustic alkali to 

 the extent of from 15 to 20 c.e. 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 litre 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 tlie filtered bouillon, neutralized 



