CHAPTER XXV 



INFLUENCE OF DISINFECTANTS ON THE GROWTH 

 OF MICRO-ORGANISMS 



EXERCISE I 



1. Prepare fifty-seven Hill's test-rods. These are 

 prepared in the following manner: Glass rods about 

 two inches longer than ordinary culture-tubes are marked 

 with hydrofluoric acid by a circle exactly one inch from 

 the end. A wad of cotton is then wrapped around the 

 middle of the rod, and this is inserted in a culture-tube. 

 The rod is then pushed down until it nearly reaches the 

 bottom. That part of the rod which is free at the upper 

 end is used for labeling. The whole apparatus is then 

 sterilized in the dry-air oven. 



2. Fill two wide-mouthed flasks, one with 100 c.c. 

 of a 5 per cent, solution of carbolic acid, the other with 

 100 c.c. of a i per cent, solution. 



3. Fill two similar flasks, one with 100 c.c. of a solu- 

 tion of mercuric chlorid i : 1000, the other with a solution 

 of i : 10,000. 



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

 per cent, solution of formalin (40 per cent, formaldehyde), 

 the other with a i per cent, solution. 



5. Prepare 48-hour broth-cultures of Staphylococcus 

 pyogenes 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 thermostat, 

 after marking each tube carefully. 



