198 LAUBENCE F. FOSTER 



acid employed in fixing the initial reaction of the medium. Fur- 

 ther doubt has been thrown upon the ''physiological constant" 

 theory by the work of Wyeth (1918) on Bad. coli. By constructing 

 ''reaction resultants" such as those suggested by Wolf and Harris 

 (1917a) he was able to show a definite relationship between the 

 initial and final Ph levels. The type of acid employed in adjusting 

 the medium was also found to bear a definite relationship to the 

 final Ph produced by the organisms. From the foregoing results 

 these investigators concluded that no method of clinical differen- 

 tiation based upon the production of a characteristic level of 

 hydrogen-ion concentration may safely be applied, unless such 

 factors as the initial Ph of the culture medium as well as its 

 composition be very carefully controlled in every test. 



Wolf and Harris (ibid.) have directed attention to the fact 

 that fermentations characterized by a slowly decreasing produc- 

 tion of acid in the period of depressed acceleration give rise 

 to a final Ph which is a constant regardless of the initial reaction, 

 provided the activities of the organism cease as soon as a definite 

 level of Ph is attained. Expressed differently, the "reaction 

 resultant" appears as a straight line parallel to the abscissa. 

 Seemingly this condition prevails in streptococcus fermentations 

 as table 10 reveals a marked constancy in the levels of final 

 Ph produced in glucose and in glucose-serum media. So far as 

 the initial reaction is concerned it must be concluded that this 

 factor is without influence upon the production of a character- 

 istic hydrogen-ion concentration but that levels of initial Ph 

 which allow growth to occur satisfactorily will also conduce to 

 the attainment of the Ph level estabhshed as a "physiological 

 constant" of Streptococcus hemolyticus. That the composition 

 of the medium may exert an effect upon the final Ph however, 

 is illustrated in the values obtained with the glucose-serum 

 series (table 10). Here there is a tendency toward the produc- 

 tion of shghtly higher points of hydrogen-ion concentration, 

 that is, lower Ph levels. 



