APPENDIX 



REACTION OF CULTURE MEDIA 



THE importance of the reaction of media as a controlling factor 

 in the development of biological characters is of so much importance 

 that the methods recommended by the committee of bacteriologists 

 appointed in 1895 to the American Public Health Association in 

 1897 are appended to aid those who may not have the transactions of 

 that association at hand. 



"The first thing t obtain is a standard 'indicator' which will 

 give uniform results. These requirements are best fulfilled by phe- 

 nolphthalein. 



This indicator was first suggested by Schultze in combination with 

 the titration method for obtaining the desired reaction for culture 

 media (Cent, fur Bakt. und Parasit., Bd. X., 1891, S. 53), but its gen- 

 eral adoption seems to have been retarded largely by Dahmen (Cent, 

 fiir Bakt. und Parasit., Bd. XII., 1892, S. 620), who claimed that its use 

 was not feasible, owing to complications which might arise from the 

 presence of carbonates and ammonium salts in the solution to be 

 tested. These objections to the use of phenolphthalein do exist, 

 but may be readily overcome. 



The amount of free and -combined ammonia present in culture 

 media at the time the reaction is determined, has been found not 

 to exceed .003 %, which is less than one-tenth the amount which 

 interferes with the accuracy of this indicator ; while the production ot 

 carbon dioxide is obviated to a very great degree by neutralizing 

 with sodium hydroxid instead of with sodium carbonate, and any 

 of this gas which may be absorbed from the atmosphere is practi- 

 cally all driven off by heat during the preparation of the media. 



The great advantage in the use of phenolphthalein over other 

 indicators lies in the fact that it takes into account the reaction of 



