DISINFECTANTS 69 



Ki t2 



K2 ti 



We have, therefore 



- (-^T- 



n = 



log ta - log ti 



log Ci - log C2 



Chick (1908) found the following death times for wash- 

 ed cells of Bacterium paratyphosum exposed to silver ni- 

 trate : 



Concentration : Death Time : 



0.085% 0.75 minutes 



0.017% 1.5 



0.0085% 2.5 



0.0017% 6.5 



0.00085% 22.5 



0.00017% 56 



0.000085% 140 

 The exponent can be computed from any two sets of data. 

 The two extremes give 



^ log 140 - log 0.75 ^^rjn 



log 1000 * ' 



The next two values give 



log 56 -log 1.5 _ ^ r,Q 

 ""= — b^lOO ^'^' 



The significance of the concentration exponents will 

 appear more clearly in a specific example. At a con- 

 centration of 0.7%, formaldehyde and phenol have about 

 the same disinfecting power and kill Bact. coli in about 

 100 minutes. If the concentration is doubled, formal- 

 dehyde, for which n = 1, kills in 50 minutes, while phe- 

 nol, with n = 6, will kill in 100/2<^ = 100/64 = 1.6 min- 

 utes. If the disinfectants are diluted with an equal vol- 

 ume of water, 0.35% formaldehyde will kill in 200 min- 

 utes while phenol requires 100 X 2'' = 6400 minutes = 

 4.5 days. 



