162 LAUEENCE F. FOSTER 



Ted test in use at the present time. Ayers (1916) in an investi- 

 gation of the final hydrogen-ion concentration in some 200 cul- 

 tures of streptococci was able to demonstrate a somewhat higher 

 acidity^ in cultures of the non-pathogenic than in those of patho- 

 genic species grown upon glucose broth. Later work by Ayers, 

 Johnson, and Davis (1918), as well as by Avery and CuUen 

 (1919a), has led to the suggestion of a rapid presumptive test for 

 the differentiation of bovine and human streptococci based 

 upon differences in the final hydrogen-ion concentration pro- 

 duced in glucose broth. However, as Brown (1920) has pointed 

 out, no single procedure can perhaps serve to differentiate the 

 two varieties absolutely inasmuch as atypical strains are some- 

 what frequent. Cullen and Chesney (1918), Jones (1920, 

 1920a), Avery and Cullen (1919b), and Lord and Nye (1919), 

 working with pneumococci of the various types in glucose broth, 

 liave found a final hydrogen-ion concentration that is in close 

 agreement with the constant established for the streptococci. 

 This value appears to be the same in all types irrespective of 

 immunological character. The works of Fred and Loomis (1917) 

 upon alfalfa bacteria, of Bunker (1916-1917, 1919) and Davis 

 (1918) upon Corynebact. diphtheriae, of Itano (1916a, 1916b) 

 upon B. subtilis and certain streptococci, of Cole and Onslow 

 (1916) upon the typhoid group, of Clark (1917) upon Lacto- 

 bacillus bulgaricus, of Waksman and Joffe (1920) upon Actino- 

 mycetes, of Ayers and Rupp (1918) upon members of the alkali- 

 forming group, of Wolf and Harris (1917a, 1917b) upon Clostridiu7n 

 welchii and C. sporogenes, of Gillespie (1916, 1918) on soil organ- 

 isms, and of Cohen and Clark (1918) upon various organisms 

 are indicative of an attempt on the part of present-day workers 

 to gain a more accurate knowledge of the metaboHc activities of 

 micro-organisms through the measurement of changes in the 

 hydrogen-ion concentration brought about in culture media. 

 Determinations of titratable acidity and ammonia, according to 

 Kligler (1916), give an indication of the extent of carbohydrate 



2 The terms acid and acidity in the present paper refer to true acidity as ex- 

 pressed in terms of Ph, except when the reference is specifically to titratable 

 acid or acidity. 



