C.-E. A. WINSLOW 79 



under the influence of heat is explained by Chick and Martin (1910) as a heat 

 coagulation consisting in a reaction between protein and water which is highly accel- 

 erated by heat. From the practical standpoint of food preservation, Bigelow (1921) 

 has shown that the logarithmic relationship between sterilizing time and temperature 

 holds for the destruction of both spores and vegetative cells by high heat. 



Phelps (191 1) has given a very valuable analysis of the variations in the effective- 

 ness of a given disinfectant with respect to concentration, time, and temperature, and 

 has shown how its constants can all be fixed by determining its efficiency in two dif- 

 ferent dilutions at the same temperature and at two different temperatures in the 

 same dilution (three determinations in all) ; the values for any other set of conditions 

 can then be obtained from the formulas: 



log^ = KCH 

 and 



when (as before) jB= initial number of bacteria present, 5 = final number of bacteria 

 present, / = elapsed time, C = concentration of disinfectant. A' = velocity constant cal- 

 culated at temperature of experiment, irr° = same at any temperature T°, Kio" — 

 same at 20°, w = concentration exponent, and = temperature coefficient. 



6. THE PHASE OF READJUSTMENT 



Toward the close of the phase of decrease the rate of mortality slackens and the 

 curve passes imperceptibly into the final phase of readjustment. In the work of 

 Chick (1908) on the effect of chemical disinfectants upon vegetative cells, and in the 

 studies by Falk and Winslow (1926) on the death-rate of bacteria in dilute salt solu- 

 tions, it even appears, as we have seen, that the value of K falls progressively through- 

 out the phase of decrease (see Fig. 4) so that there is no clear distinction between the 

 two processes. In other instances, however, it is often possible to observe two fairly 

 distinct periods, one of rapid and one of slow decline. Thus, Winslow and Cohen (1918) 

 in their studies of the life of Bact. coli in water observed values of K ranging from .004 

 to .020 for the first ten days while from the tenth to the sixtieth day the values varied 

 only between .001 and .002. 



The mortality curves given by Sedgwick and Winslow (1902) for the life of ty- 

 phoid bacilli in ice and in dry earth, by Winslow and Abramson (191 2) for colon bacilli 

 in water, all show a falling rate of mortality toward the close of the cycle of decrease. 

 Values for K calculated from these data are cited below, obtained by merely sub- 

 tracting the log of the number present after a given time interval from the log of the 

 number present at the beginning of that interval and dividing by the elapsed time. 

 (It should be noted that in computing these values the reduction in numbers for each 

 period are computed on the basis of the number present at the beginning of that pe- 

 riod instead of following the usual procedure of computing K on the basis of reduction 

 from the beginning of the whole experiment. The latter method naturally tends to 

 obscure any differences which may occur.) 



The very gradual rate of decrease during the phase of readjustment may end in 



