83 



come just above the liquid in the tubes. Remove the tubes, 

 one of each species, as follows. One in 5 minutes, one in 10 

 minutes, one in 15 minutes and one in 20 minutes. Isabel 

 and place them in the incubator. 



At the next exercise, examine the heated tubes and note 

 which are clear and which contain a growth. If the tubes 

 heated for 10 minutes or longer have a growth, repeat the 

 experiment at 70° C. If this fails to destroy them repeat at 

 80° C. and if necessary apply a still higher temperature. 



Examine the cultures microscopically in all the fertile 

 tubes to determine if they are pure. 



Explain the cause for the difference in the thermal death 

 point between the two organisms. 



Inoculate for the next exercise a tube of bouillon from 

 each of the cultures of M. {Staph.) pyogenes aureus and B. 

 subtilis furnished. 



EXERCISE lylll. 

 DETERMINING THE EFFICIENCY OF DISINFECTANTS. 



§ 147. Explanatory note. The efficiency of the more 

 commonly used disinfectants has been determined for most 

 of the pathogenic bacteria, but new disinfectants are con- 

 stantly being put upon the market, and before it is safe to 

 use or recommend them, their efficiency should be deter- 

 mined. With many of the disinfectants, such as carbolic 

 acid, corrosive-sublimate, lime, and the mineral acids, much 

 stronger solutions are commonly used than are actually 

 necessary to kill the bacteria, owing to the fact that fre- 

 quently it is necessary to allow for an indefinite waste due to 

 the union of the disinfectant with other substances, usually 

 organic, with which the 'bacteria are mixed. For the differ- 

 ent methods of testing the efficiency of disinfectants, see 

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



