292 CHEN CHONG CHEN AND LEO F. RETTGER 



bation temperature too low, colorimetric determinations with 

 the methyl red indicator may be positive and correlation with 

 the other tests will be lacking. 



Difco peptone-phosphate-glucose medium is less well adapted 

 for the study of hydrogen ion concentration of the aerogenes 

 type than the synthetic and the Witte peptone media of Clark 

 and Lubs, and a greater proportion of failures to correlate must 

 be expected, expecially if the exacting conditions as to length 

 of incubation and incubation temperature are not supplied. The 

 difference appears, however, to be chiefly one of degree rather 

 than kind. 



The Voges and Proskauer reaction has proven itself in the 

 present investigation to be of very great value in differentiating 

 between the coli and aerogenes types of bacteria. Neither the 

 character of the medium nor the period of incubation seems to 

 interfere with the carbinol formation. A colorless medium is as 

 important, however, for this test as for the colorimetric deter- 

 mination of the hydrogen ion concentration, owing to the fact 

 that, while the characteristic coloration of the tubes cannot be 

 mistaken, it may be at times obscured by coloring matter in the 

 medium itself, especially if the reaction is relatively weak. Fur- 

 thermore, intelligent execution of the test is absolutely necessary. 



The Voges and Proskauer reaction should be observed within 

 shorter incubation periods than have been customary. A weak 

 reaction persists only for a short time, while a strong reaction may 

 last for several days, with a gradual fading away of the color. 

 A period of from two to eight hours may be regarded as sufficient 

 for a positive V and P reaction, if correctly carried out. Abun- 

 dant oxygenation of the medium after the addition of the alkali, 

 and proper incubation are important factors. 



The uric acid test of Koser constitutes another reliable cor- 

 relation test. Great care must be exercised, however, in the 

 employment of test tubes, etc. which are free from adhering 

 nitrogenous matter. Koser obtained excellent results with uric 

 acid and hypoxanthine. In the present work xanthine, theobro- 

 mine and caffeine were employed besides uric acid, the xanthine 

 and uric acid giving very gratifying results. No growth could 



