COLON-AEROGENES GROUP OF BACTERIA 291 



little value, also, and should be used with caution. While all of 

 the B. coli strains produce indol, the percentage of positive 

 tests with the aerogenes type is too large to make the test a prac- 

 tical one. The Ehrlich method of determining indol formation 

 is decidedly more delicate than the Salkowski sulphuric acid 

 test, and must be regarded as the more reliable. In the present 

 investigation gelatin liquefaction has been .of no value as a test. 



The constancy of the hydrogen ion concentration in cultures 

 of the B. coli type, as determined by the colorimetric method, is 

 proved beyond doubt by the present study. The hydrogen ion 

 concentration can be adequately measured for practical purposes 

 by the methyl red indicator, as Clark and Lubs have claimed. 

 The synthetic medium of Clark and Lubs appears to be best 

 suited for the colorimetric determination of acidity. Witte's 

 peptone-phosphate-glucose medium of Clark and Lubs answers 

 the purpose well for which it is intended provided that it is kept 

 colorless or practically free from color during the process of 

 sterilization. 



Owing to the interference of simultaneous alkali production 

 the limiting hydrogen ion concentration of the aerogenes strains 

 could not be so easily determined. The results of the present 

 investigation show that the hydrogen ion concentration of these 

 strains is completely masked after five days' incubation both in 

 the synthetic and in the Witte peptone-phosphate-glucose 

 medium. 



A new set of experiments was conducted to determine daily 

 the hydrogen ion concentration of a collection of 60 aerogenes 

 strains in the synthetic medium for a period of three weeks, in 

 order to ascertain the degree of OH-ion concentration reached 

 by the cultures. The H-ion concentration began to decrease 

 at the end of three days of incubation, and the pH value of the 

 Otl-ion concentration proceeded to increase steadily and pro- 

 gressively until the end of the experiment. The greatest H-ion 

 concentration was 4.7, and the highest OH 7.4. It must be ap- 

 parent, therefore, that length of incubation and temperature are 

 two very important factors in the interpretation of results. If 

 the colorimetric determination is made too soon, or the incu- 



