FACTORS MODIFYING DISINFECTION 631 



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 

 a considerable influence on the extent or rate of disinfec- 

 tion. Up to the optimum temperature at which the 

 organism to be disinfected grows on the medium in which 

 it is exposed the activity of a disinfectant may fall off 

 as the temperature rises, owing to the increased vigour 

 which the organism derives from the improvement in 

 its conditions in respect of temperature. A relatively 

 small difference of temperature two or three degrees 

 may make an appreciable difference in the activity of the 

 disinfectant, and in the examination of disinfectants the 

 failure to remember this fact has led to serious error. 

 Above the optimum a rise of temperature increases the 

 activity of the disinfectant, sometimes to an enormous 

 extent. The same is sometimes the case even at tem- 

 peratures below the optimum, when the organism is in 

 unfavourable conditions for growth. A mixture of dis- 

 infectants in many cases has a more powerful effect than 

 can be produced by either separately (Chamberland). The 

 resistance of bacteria to disinfection by chemical agencies 

 is extremely variable and is also selective. Bacteria of 

 one class may be many times more sensitive to one 

 disinfectant than to another when both substances 

 exert an equal effect on bacteria of another class. The 

 presence of organic matter may profoundly modify the 

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



