66 



between the incidence of the colon group in drinking water and the ty- 

 phoid fever rate in the community. 



Viability of Bact. coli and Bad. typhi in Water. These organ- 

 isms do not find favorable conditions for growth in natural waters. The 

 food supply is apparently insufficient or unavailable, the temperature is 

 unfavorable, other organisms exert a deleterious effect, and in consequence 

 a diminution in number rapidly ensues. It is conceivable that under cer- 

 tain conditions of extremely high temperature, as may occur in summer, 

 there may be an initial increase but death soon results. A convenient 

 method for appraising the relative viability of two organisms is to com- 

 pare the rates at which they die off under a given set of conditions. 



The rate of death of bacteria may be expressed by 



1 ' N 



K z 77^0 lo T where ; 



"N" is the number of organisms at the beginning, i. e. time "To". 



"n" is the number of organisms at the end of the time, "T^'. 



"K" is a measure of the rate of death. 



If the logarithms of the number of surviving bacteria at different 

 periods are plotted against the elapsed time it will be observed that the 

 points fall along a straight line intersecting the "x" axis at an angle whose 

 tangent is numerically equivalent to "K". The greater the rate of death 

 the greater will be "K". In the following tables are given the number 

 of surviving Bact. coli and Bact. typhi when stored in water and exposed 

 to the air. It will be observed that the rate of death of Bact. typhi is 

 considerably higher than Bact. coli, though the latter organism also dies 

 off rather rapidly. 



TABLE XXIX. VIABILITY OF BACT. 



COLI IN WATER AT 2QO C. 



(After Hinds 1916) 



TABLE XXX. VIABILITY OF BAC1 

 TYPHI IN WATER AT 2QO C. 

 (After Hinds 1916) 



Average "K" 0.094 



Average "K" 0.87 



The writer made some observations at the Massachusetts Institute of 

 Technology in 1913 on the effect of temperature on the viability of Bact. 

 coli in distilled and conductivity water containing C. P. salt. The results 

 are shown in figures 3 and 4 where it is apparent that the higher the tem- 

 perature the greater the rate of death, and that the presence of gases in 

 solution also affects the death rate. 



