696 LEON S. MED ALIA 



A criticism (Gillespie, 1921) of this method of preparing color 

 standards based on theoretical mathematical calculations has no 

 weight, if the colors, actually prepared as described, do actually 

 match other solutions of known pH. The calibration of each 

 "pair" in the author's series has been made by such actual com- 

 parisons, and the pH values assigned are based upon actual 

 visual agreements with known standards and not from hypothet- 

 ical relations. 



As to the criticisms brought against using these pH measure- 

 ments for the study of acid production by bacteria, it is the 

 author's point of view, that the value of being able to study from 

 hour to hour the change produced in media by a given culture 

 without having to disturb the sterility of the culture, and the 

 ability to measure such change in reaction in either direction in 

 such a simple way that can be dupUcated, is beyond measure. 

 From that point of view, it is not the question of "how much" 

 acid is produced, in an abstract way, in which we are interested; 

 but rather the change in reaction, produced, biologically, in a 

 culture medium suited for the growth of a particular organism, in 

 a relative way. This is of particular value if it can be done with- 

 out contaminating the culture or introducing foreign elements. 



It is important that the experimenter state, however, what 

 method is used in determining acid or alkali production by bac- 

 teria, and for that matter in making use of the pH measurement 

 for any other purpose. Tliis will avoid confusion when the 

 work is dupUcated by others. 



In concluding I wish to express my appreciation to Prof. S. C. 

 Prescott, and Prof. J. W. M. Bunker, and Dr. jNI. P. Horwood 

 of the Department of Biology and Public Health of the Institute 

 for their helpful assistance and criticism rendered me in this 

 work. 



REFERENCES 



Barnett, G. D., and Chapman, H. S. 1918 Colorimetric determination of 

 reaction of bacteriologic mediums and other fluids. Jour. Amer. 

 Med. Assoc, 70, 1062-1063. 



BuNKEH, George C, and ScHUBER, Henrt 1922 "The Reactions of Culture 

 Media." Jour. American Waterworks Assn., 9,63-116. 



