APPENDIX 
REACTION OF CULTURE MEDIA 
THE importance of the reaction of media as a controlling factor 
in the development of biological characters is such that the methods 
recommended by the committee of bacteriologists appointed in 
1895 to the American Public Health Association in 1897 are ap- 
pended to aid those who may not have the transactions of that 
association at hand. 
“The first thing to obtain is a standard ‘indicator’ which will 
give uniform results. These requirements are best fulfilled by 
phenolphthalein. 
This indicator was first suggested by Schultze in combination 
with the titration method for obtaining the desired reaction for 
culture media (Cent. fiir Bakt. und Parasit., Bd. X., 1891, S. 53), 
but its general 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 produc- 
tion of carbon dioxid is obviated to a very great degree by 
neutralizing with sodium hydroxid instead of with sodium carbon- 
ate, and any of this gas which may be absorbed from the atmos- 
phere is practically 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 
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