EXPERIMENTS 295 



c) Repeat experiment, using 1 drop of milk (about $ cu. 



cm.). 



d) Melt 2 tubes of gelatine and pour into Petri dishes. Ex- 

 pose one to air at beginning of school ; the other, after sweeping 

 or dusting. Cover and allow to develop as before. 



e) Prepare 2 Petri dishes and allow to cool. Touch to sur- 

 face of gelatine in one dish some part of skin, as a finger tip, 

 which has not been washed for several hours. Touch to surface 

 of gelatine in other dish the same finger tip immediately after it 

 has been washed in soap and warm water. Compare the 

 growth after 24 and 48 hrs. 



8) Effect of heat (sterilization) upon bacteria: 



a) Cut some raw meat into small bits. Place a few pieces 

 in a test tube and cover with 2 in. of lukewarm water. Plug it 

 with a tuft of cotton and set aside in a warm place. 



Place an equal number of pieces in another test tube and 

 boil briskly for a half minute. Plug and set aside with former 

 test tube. 



At end of 24 and 48 hrs., compare the 2 tubes and note effect 

 of heat. 



b) Place in 2 test tubes 2 in. of milk. Boil one by placing 

 it in a small vessel of boiling water for 5 min. Plug both tubes 

 with cotton and put together in a warm place. Compare at 

 end of 24 and 48 hrs., testing acidity and odor. 



9) Effect of disinfectants upon bacteria: 



a) With scissors cut the white of an egg into 10 times its 

 weight of water. Put 2 in. of the solution into each of 14 test 

 tubes. To No. 1, add nothing. To No. 2, add 4 grs. of salt; 

 to No. 3, 15 grs. To No. 4, add 15 grs. of sugar; to No. 5, 

 45 grs. To No. 6, add 2 drops of 1 to 1000 corrosive sublimate 

 solution ; to No. 7, 6 drops. To No. 8, add 1 drop of formalin, 

 40 per cent solution ; to No. 9, 2 drops ; to No. 10, 3 drops. To 

 No. 11, add 2 grs. of borax ; to No. 12, 4 grs. To No. 13, add 

 4 drops of 5 per cent carbolic acid solution ; to No. 14, 10 drops. 



Shake the 14 test tubes carefully to mix contents, plug firmly 

 with cotton, and set aside in a warm place. Examine daily and 

 note changes in each, in order to determine relative effectiveness 

 of different mixtures as preservatives. 



