314 J. HOWARD MUELLER 



Determination of liyniling hydrogen-ion concentration for strains 



employed 



It was found, by inoculating a series of plain broth media with 

 pH ranging from 6.8 to 8.6, that the pneumococci found 

 conditions suitable for growth between pH 7.4 and 8.0, while 

 the hemolytic streptococcus grew well throughout the same 

 range, and even as far down as pH 6.8. For the adjustment of 

 experimental media, therefore, a reaction of pH 7.4 to 7.8 has 

 been used for all the strains of test organisms. In cases where 

 several experimental lots of media are prepared at one time, the 

 adjustment of the reaction is facihtated by the direct addition 

 of phenol red to the entire lot. It is a simple matter to reach 

 the particular pH desired bj^ using a solution of phenol red of 

 0.02 per cent concentration, in the proportion of 1 ce. to 25 cc. 

 of media. Normal or semi-normal NaOH (moderatelj^ free from 

 carbonate) is then added drop bj' drop until the color is distinctly 

 red, but not purple. One can quicklj- learn to recognize the 

 correct color without the need of standard color tubes. In the 

 case of unusually dark colored media the reaction can be ad- 

 justed approximately by this means and finished by the usual 

 method of dilution and addition of more indicator followed by 

 comparison with standard tubes. Experiment showed that 

 several times this amount of phenol red could be added to media 

 without any influence on the growth of the organisms in question. 



Limiting range of osmotic pressure 



Although moderate variations in osmotic pressure are prob- 

 ably of httle importance in culture media, an experiment was 

 carried out in broth made up with varjing concentrations of 

 NaCl. The test organisms grew equally weU in salt-free broth 

 and through several intermediate concentrations up to an equiva- 

 lent of 2 per cent NaCl, so that with ordinary care the question 

 of osmotic pressure apparently need not enter into consideration. 



Presence of glucose 



In preliminary work with the pneumococci it was observed 

 that occasionally a lot of meat infusion was met with which 



