630 A MANUAL OF BACTERIOLOGY 



caution should be exercised in adding salts, etc., to increase 

 solubility or prevent precipitation, as the addition may 

 seriously impair germicidal or antiseptic power (see p. 635). 

 The disinfection process is a gradual one. In the early 

 stages of disinfection large numbers of organisms are killed, 

 but the rate of killing becomes slower and slower as time 

 elapses. Madsen and Nyman and Miss Chick 1 have found 

 that if the results be plotted, ordinates representing the 

 numbers of surviving bacteria, and abscissa the corre- 

 sponding 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 



1 n 



by the formula - - log - = K, where % and n 2 are 

 t 2 -t & ^ 2 



the numbers of bacteria surviving after times t and t 2 

 respectively, and K is a constant. In the case of disinfec- 

 tion 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 resistant than the younger. 

 The progress of heat disinfection apparently follows the 

 same course. Miss Chick asserts that the act of disinfec- 

 tion is a unimolecular reaction, but it is difficult to accept 

 this view. Disinfectants in emulsion tend to be more 

 efficient than when in solution. 



Factors modifying disinfectant action. 2 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 



1 Journ. of Hygiene, vol. viii, 1908, p. 92 (Summary and Bihliog.). 



2 This section is largely taken from Applied Bacteriology, Moor and 

 Hewlett, 1907. 



