184 BAENETT COHEN 



pH zone of tolerance, or minimum death rate, of Bad. typhosum 

 lies between 5.0 and 6.4; that for Bact. coli is wider, and centered 

 near absolute neutrality. The effect of sub-lethal factors is 

 to make a period of induction apparent before death proceeds 

 at a logarithmic rate. 



The phenomena of growth, maintenance and death of bacteria 

 are fundamentally important to bacteriology, theoretical and 

 applied; and there are conceivably numerous factors that may 

 influence them. Omitting from consideration in this discussion 

 the obviously large part played by the food supply, we recognize 

 certain predominating physical and physico-chemical factors, 

 chief among which are temperature and the concentration of 

 hydrogen ions. In bacterial death, and in disinfection, which 

 may be regarded as a special phase of bacterial death, their 

 role may indeed become paramount. 



Temperature plays its important part in the Ufe and death 

 processes of bacteria by controlling the active agencies involved. 

 Referring to the death processes Clark and Lubs (1917) have 

 said: ". . . . in cellular destruction temperature is to be 

 considered as an accelerating condition .... among the 

 active agents concerned the concentration of hydrogen ions may 

 be of great significance." This opinion is amply supported by 

 experimental evidence reaching back to the days of Pasteur. 



As an aid to the study of certain phases of biological processes, 

 the effect of temperature has been widely used and has yielded 

 facts of importance. It has been found empirically that tem- 

 perature augments physical phenomena in arithmetical progres- 

 sion and chemical phenomena in geometric ratio. As a rule the 

 temperature coefficient of velocity of most chemical reactions 

 for a rise of ten degrees is 2 or more ; and for physical phenomena 

 it is nearer 1. 



Snyder (1908, 1911) cites numerous examples of these two 

 types of temperature coefficient and shows that in most physio- 

 logical processes the temperature coefficient of chemical reaction 

 velocity appUes. Loeb (1908) ingeniously accounts for the 

 much richer animal and plant life of Arctic waters by reasoning 

 that since a temperature decrease of ten degrees increases via- 



