138 



DISINFECTION 



which, 

 comes 



on integration, be- 



1 



k = - log 



t a —X 



This is the equation repre- 

 senting the velocity of a uni- 

 molecular reaction and is 

 often spoken of as the loga- 

 rithmic law. If log (a —x) be 

 plotted against time in this 

 equation, the resulting graph 

 will be a straight line (Fig. 

 22). 



We may adapt the uni- 

 molecular reaction definition 

 to the process of disinfection 

 by saying that at any moment 

 the reaction velocity is pro- 

 portional to the number of 

 surviving bacteria per unit volume. For example, let us suppose that there are 

 100,000 organisms being submitted to disinfection, and that the rate is one at 

 which 90 per cent, of the organisms are killed in each minute. Then : 



Fig. 22, 

 Disinfection of anthrax spores with 5 per cent, phenol at 

 33-3° C. The curve is drawn through a series of cal- 

 culated points ; the circles represent the experimental 

 observations. This curve is constructed from the same 

 observations as those used for Fig. 21, but the num- 

 bers of organisms are expressed logarithmically. 

 (After Chick.) 



Time. 

 After minutes 

 „ 1 

 „ 2 

 „ 3 

 „ 4 



Nos. Surviving. 



100,000 

 1/10 X 100,000 or 10.000 

 1/10 X 10,000 or 1,000 ■> 

 1/10 X 1,000 or 100 

 1/10 X 100 or 10 



Supposing that B represents the initial number of living organisms, and b the 

 final number, then the reaction velocity may be expressed by the equation : 



7. 1 1 B 



i=-log- 



Chick, using the drop method, made experiments on the disinfection of anthrax 

 spores by 5 per cent, phenol. Her results are given in Table 15 and Figs. 21 



and 22. 



TABLE 15 

 Anthrax Spores. 5 per cent. Phenol. 33-3° C. 



^ Values of k are calculated to base 10 and not to base e, and in hours, not in minutes as in 



Table 18. 



