DISINFECTION STUDIES 



203 



Our curves being derived under the same conditions are com- 

 parable and their relations may be designated numerically by 

 their slopes, provided the curves are fairly straight lines, as in 

 the present case. This is another way of stating that we are 

 comparing rates of decHne, or reaction velocities if we prefer to 

 think in terms of the monomolecular reaction. The velocity 

 constants thus derived are given in table 3. 



Thb mode of expressing differences in disinfection has many 

 advantages and has been recommended by Phelps (1911) as 

 one of the logical procedures in the evaluation of disinfectants. 

 Buchanan (1918), Bruett (1919) and Salter (1921) have utilized 

 it in studies of heat sterilization and thermal death points. 



TABLE 3 

 Average velocity constants for the death of Bact. typhosum at different pll values 



at 20°C. 



* Relative k represents the magnitudes of the various constants compared with 

 that at pH 5.4 taken as unity. 



The velocity constants we have thus derived show clearly 

 that there is a zone between pH 5.0 and 6.4 in which Bact. ty- 

 phosum declines in numbers very slowly. A small change in 

 pH toward the more acid side of the zone produces an almost 

 hundred-fold increase in the rapidity of death; while on the 

 alkaline side a similar change in pH increases the death rate 

 only four or five times. In brief, a small increase in the concen- 

 tration of hydrogen ions at the acid end of the zone of tolerance 

 produces a profound effect, whereas relatively large increases 

 of hydroxyl ions produce a much smaller effect. This coincides 

 with the findings of Cohen and Clark (1919) who called atten- 

 tion to a similar effect upon the growth of members of the colon 

 group of bacteria. 



This observation is of some importance theoretically, and may 

 help to throw light upon the mechanism involved in the death of 



