PHYSICAL AGENTS 119 



surgeon "sterilizes" his instruments, but "disinfects" his 

 operating table and room. The dairy- workers mentioned, 

 above steriHze their apparatus, pails, milk bottles, etc. 

 Evidently the object of the two processes is the same, remov- 

 ing or destroying living organisms, the name to be applied 

 is largely a question of usage and circumstances. Any agent 

 which is used to destroy microorganisms is called a "disin- 

 fectant." Material freed from living organisms is "sterile." 



The process of preventing the growth of organisms without 

 reference to whether they are killed or removed is spoken 

 of as "antisepsis," and the agent as an antiseptic. Hence a 

 mildly applied "disinfectant" becomes an "antiseptic," 

 though it does not necessarily follow that an "antiseptic" 

 may become a disinfectant when used abundantly. Thus 

 strong sugar solutions prevent the development of many 

 organisms, though they do not necessarily kill them. 



Asepsis is a term whicji is restricted almost entirely to 

 surgical operations and implies the taking of such precau- 

 tions that foreign organisms are kept out of the field of opera- 

 tion. Such an operation is an aseptic one, or performed 

 aseptically. 



A "deodorant or deodorizer" is used to destroy or remove 

 an odor and does not necessarily have either antiseptic or 

 disinfectant properties. 



The agents which are used for the above-described pro- 

 cesses may be conveniently divided into physical agents and 

 chemical agents. 



PHYSICAL AGENTS. 



1. Drying.^-This is doubtless the oldest method for pre- 

 venting the growth of organisms, and the one which is used 

 on the greatest amount of material at the present time. A 

 very large percentage of commercial products is preserved 

 and transportted intact because the substances are kept free 

 from moisture. In the laboratory many materials which 

 are used as food for bacteria (see Chapter XVI) "keep" 

 because they are dry. Nevertheless, drying should be con- 

 sidered as an antiseptic rather than as a disinfectant process. 

 While it is true that the complete removal of water would 



