THEORY OF DISINFECTION 759 



the corresponding times, the points lie on a logarithmic 

 curve. The curve so obtained, in fact, appears to be 

 similar in form to that of a " unimolecular reaction," and 



may be expressed by the formula - log I = K, where 



t% t-i n z 



HI and 712 are the numbers of bacteria surviving after 

 times / and 2 2 respectively, and K is a constant. In the 

 case of disinfection of anthrax spores with phenol, Miss 

 Chick found the mean value of K to be 0-44. In the case 

 of B. paratyphosus , however, the course of the disinfection 

 is different unless the culture is very young, and Miss 

 Chick concluded that the older individuals are less resis- 

 tant than the younger. The progress of heat disinfection 

 apparently follows the same course. Miss Chick asserts 

 that the act of disinfection is a unimolecular reaction, but 

 it is difficult to accept this view. Disinfectants in emul- 

 'sion tend to be more efficient than when in solution. 



Factors modifying disinfectant action. The efficiency of 

 a disinfectant liquid partly depends on its concentration. 

 The rate of penetration into bacterial cells decreases as 

 the concentration increases above a certain limit. Most 

 disinfectants yield, therefore, a greater amount of disin- 

 fectant energy per gramme-hour in dilute than in strong 

 solutions. In oil, glycerin, or alcohol, disinfectants lose 

 some or most of their activity. Spores in anhydrous 

 glycerin, oil, or vaseline, are not killed at a temperature 

 lower than 170 C. acting for half an hour. 1 Of fats, 

 lanolin alone seems compatible with disinfectant efficiency. 

 Some disinfectants form an emulsion on the addition of 

 water, and their efficiency for a given amount of active 

 material may vary within wide limits according to the 

 manner in which they are emulsified. The temperature 

 at which the organism is exposed to the disinfectant has 



1 Bullock, Jovrn. of Hygiene, xiii, 1913. p. 168. 



