CHAPTER XV. 

 DISINFECTION AND STERILIZATION (Continued). 



CHOICE OF AGENT. 



The choice of the above-described agents depends on the 

 conditions. Evidently a barn is not to be disinfected in the 

 same way that a test-tube in the laboratory is sterilized. 

 Among the factors to be considered in making a choice are 

 the thing to be disinfected or sterilized, its size and nature, 

 that is, whether it will be injured by the process proposed, 

 cost of the agent, especially when a large amount of material 

 is to be treated. Among the conditions which affect the 

 action of all agents the following should be borne in mind 

 particularly when testing the disinfecting power of chemical 

 agents: 



1. The kind of bacterium to be destroyed, since some are 

 more readily killed by a given disinfectant than others, even 

 though no spores are present. 



2. The age of the culture. Young bacteria less than twenty- 

 four hours old are usually more readily killed than older ones 

 since the cell wall is more delicate and more easily pene- 

 trated, though old growths may be weakened by the accu- 

 mulation of their products and be more easily destroyed. 



3. Presence of spores, since they are much more resistant 

 than the growing cells. 



4. Whether the organism is a "good or bad" growth, i. e., 

 whether it has grown in a favorable environment and hence 

 is vigorous, or under unfavorable conditions and hence is 

 weak. 



5. The number of bacteria present, since with chemical 

 agents the action is one of relative masses. 



6. Nature of the substance in which the bacteria are. 

 Metallic salts, especially bichloride of mercury, are precipi- 

 tated by albuminous substances and if employed at all must 



