DISINFECTANTS 105 



considered equally correct whether the bacteria are 

 killed by 1.11% phenol in 14 minutes or by 1.0% in 11 

 minutes. This means a fluctuation in the deathrate 

 constant the range of which can be calculated. In our 

 standard equation 



Ktc" = log ii^itial number 

 survivors 



the ratio of the initial number of bacteria to that of the 

 survivors is constant for any set of experiments and 

 therefore, for any combination of time and concentra- 

 tion. We have, then, 



Substituting the above limiting values of the F.D.A. 

 method for t and c, and taking 6 as the concentration 

 exponent of phenol, we find 



Ki X 11 X 1.0« = K2 X 14 X l.ir 

 and, Ki=2.4K2. 



The deathrate constant in one experiment may be 2.4 

 times as high as in the other. 



We must further consider that the killing time by the 

 unknown disinfectant must be more than 5 and less than 

 10 minutes. Sterility may be reached in one case in 6 

 minutes, in another case in 9 minutes. This increases 

 the range of the deathrate by 9/6 or 1.5, so that in two 

 standard experiments, one may have been carried out 

 with a deathrate constant which was 1.5 X 2.4 = 3.6 

 times as large as in the other experiment. Any experi- 

 ment having a greater error, i.e., greater than 360%, is 

 discarded as not being carried out under standard con- 

 ditions. This is the error which the specified conditions 

 permit and to which must be added the personal error 

 of the experimenter. 



That phenol coefficients are by no means precisely de- 

 fined values may be seen from Table 20, which shows that 

 two different authors obtained for mercurochrome phenol 



