GENERAL BACTERIOLOGY 95 



4. Fill two similar flasks, one with 100 c.c. of a 10 

 per cent solution of formalin (40 per cent formalde- 

 hyd), the other with a i per cent solution. 



5. Prepare 48-hour broth cultures of Staphylococcus 

 aureus, Bacillus coli, and B. typhosus from stock 

 cultures. 



6. Dip nineteen of these rods into each of these 

 cultures respectively, to the depth of one inch; set 

 them aside in their tubes to dry over night in the ther- 

 mostat after marking each tube. 



EXERCISE 2 



We have now six flasks containing different solutions 

 of disinfectants. 



1. Place in each one of these flasks nine of the pre- 

 pared rods, three of which have been dipped in the 

 Staph. aureus culture, three in the B. coli culture, and 

 three in the B. typhosus culture. 



2. Take three rods (one of each organism) out of 

 eacri flask after the lapse of half a minute, wash by 

 pouring sterile physiological salt solution over them 

 into a dish containing mercuric chlorid solution i : 1,000, 

 and place each rod in a tube of sterile broth. 



3. Repeat the proceedings of step 2 with a second 

 series of rods after 2 minutes. 



4. Repeat again after 5 minutes with the remaining 

 series. 



5. Place all tubes (fifty-seven) in the thermostat. 

 Three of these tubes have not been dipped into any one 

 of the six flasks containing antiseptics, and are incu- 

 bated with the others as controls. 



6. Observe the results on each of the four successive 



