8 4 



148. General directions. Prepare 20 c. c. of each of a 

 .25, .10, .02% solutions of formalin (40% formaldehyde in 

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

 each. Use distilled water in making the dilutions. Add to 

 each of the tubes in one set ^ c. c. of the bouillon culture 

 of Micrococcus {staphy ) pyogenes. And 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 dis- 

 infectant. 



Inoculate a tube of bouillon with six loopfuls from each 

 of these tubes after the expiration of the following periods 

 of time, viz. : i min., 5 min., 10 min., 20 min., and 3omin. 

 In making these inoculations allow the loop to go to the bot- 

 tom of the inoculated tube. Label each inoculated tube with 

 the strength of the disinfectant and time of exposure and 

 place it in the incubator. It should be noted that the adding 

 of ^ c. c. of culture diluted slightly the strength of the dis- 

 infectant. 



Note at the next exercise the condition of each inoculated 

 tube. From them the approximate strength and time for 

 the disinfectant to destroy the bacteria can be determined. 

 When this is found the more exact strength and time can be 

 determined by repeating the experiment with weaker dilu- 

 tions or shorter exposures or both. 



EXERCISE UV. 



TESTING DISINFECTANTS (continued). 



149. General directions. Prepare 2 sets of tubes con- 

 taining 10 c.c. each with 2.5, i.o and 0.5% solutions of 

 carbolic acid respectively, and test their effect upon the 

 organisms used in the last exercise and by the same method. 

 Note the conditions of the inoculated tubes at the next 

 exercise. 



