THE EFFICIENCY OF DISINFECTANTS III 



EXERCISE LVII 

 DETERMINING THE EFFICIENCY OF DISINFECTANTS 



166. The efficiency of the more commonly used disinfect- 

 ants has been determined for most of the pathogenic bacteria, 

 but new disinfectants are constantly being put upon the mar- 

 ket, and before it is safe to use or recommend them their effi- 

 ciency should be determined. With many of the disinfectants, 

 such as carbolic acid, corrosive sublimate, lime, and the min- 

 eral acids, much stronger solutions are commonly used than 

 are actually necessary to kill the bacteria, owing to the fact 

 that frequently it is necessary to allow for an indefinite waste 

 due to the union of the disinfectant with other substances, usu- 

 ally organic, with which the bacteria are mixed. For the dif- 

 ferent methods of testing the efficiency of disinfectants, see 

 text-books. A very simple process is given here. 



It may be desirable for students to work in groups of two or 

 more in order to economize in the number of tubes required 

 in this exercise. If possible, however, each student should 

 make all of the tests. 



References. — Young, Notes on Disinfectants and Disinfection, 

 Augusta, 1898. Rideal, Disinfection and Disinfectants, London. 

 See also text-books. 



167. Work for this Exercise — Prepare 20 c.c. of each of a 

 .25 and .10% solution of formalin (40% formaldehyde in 

 water), and distribute them in sterile tubes, putting 10 c.c. in 

 each. Use distilled sterilized water in making the dilutions. 

 Add, by means of a sterilized pipette, to each of the tubes in 

 one set ^ c.c. of a bouillon culture of B. cholera suis or of B. 

 typhosus. Add to each of the tubes in the other set the same 

 quantity of the culture of B. subtilis. 



Use a sterile pipette for adding the culture to the disin- 

 fectant. 



