94 



DISINFECTANTS 



viously given by the present writer on the basis of theo- 

 retical considerations. A fairly large number of mole- 

 cules of the cell surface must be destroyed to produce 

 an injury from which the cell cannot recover. The 

 theoretical survivor curve when death results from the 

 inactivation of many molecules agrees in every respect 

 with that observed in chlorine disinfection. 



No definite value of the concentration exponent of 

 chlorine can be obtained because of the prompt reaction 

 of this substance with so many organic compounds. It 

 can be computed from graphs like that of Figure 20 by 



Fig. 20. Semi-logarithmic plot of survivor curves of Bacterium coH 

 treated with chlorine. Abscissa: time of exposure; ordinate: per cent 

 of survivors, on logarithmic scale. (From data of Mallmann and 

 Audrey, 1940.) 



estimating the time required to kill the same percentage 

 of bacteria by different concentrations. A horizontal line 

 at 10% survivors gives ditferent results than one at 1% 

 survivors. The concentrations in this graph are in the 

 ratios 1 :2 :3 :4 :5 the corresponding times to kill 90% of 

 the cells are 140, 92, 70, 62, and 22 minutes, and the re- 

 spective concentration exponents are 1.66, 1.61, 1.69, and 

 0.87. From the times required to kill 99% of the cells, 

 the concentration exponents are 2.1, 1.67, 1.70, and 0.93. 

 The relation between concentration and death time, from 



