108 DISINFECTANTS 



ually sensitive on this particular clay. The F.D.A. 

 method discards such data as abnormal, and they do not 

 appear in the reports. But Chick's unexpurgated data 

 are used as proof that deathrate constants fluctuate too 

 much to be of value. The establishment of limits of sen- 

 sitivity for the phenol coefficient proves clearly that the 

 same fluctuations occur there, but they are never men- 

 tioned in publications. As has been said before, if we 

 consider the phenol coefficients more accurate than the 

 deathrate constants, we are merely fooling ourselves. 

 They appear more accurate only because the larger de- 

 viations are not published. 



As a result of this anathema on the deathrate constant, 

 few data can be found in the literature. Watkins and 

 Winslow (1932) give four sets of data on the death of 

 Bacterium coli by alkali. Each set concerns a different 

 cell age and consists of 4 single experiments. The high- 

 est and lowest values of K in each set are: 



0.119 and 0.166, the highest being 40% larger than the lowest 



0.136 and 0.191, " " " 40% 



0.199 and 0.242, " " " 22% 



0.336 and 0.478, " " " 42% 



This proves that with our present knowledge of the in- 

 fluence of pH, organic matter, cell concentration, culture 

 medium and age of the cells, which was not available in 

 1911 when Phelps proposed the standardization of disin- 

 fectants by death rates, the deathrate constants can be 

 measured as accurately as the phenol coefficients. They 

 would have the advantage of telling the efficiency of a dis- 

 infectant directly, and not merely from comparison with 

 another disinfectant such as phenol. 



Finally the attention of experimenters should be called 

 to a common source of error in any method which in- 

 volves the determination of a death time. It is not al- 

 ways realized that a culture showing no growth when 

 transferred to broth may not be sterile. ''No Growth" 

 means that no living cell remained in the one loopful 



