58 BACTERIOLOGICAL ANALYSIS. 



tubes, and sterilize these for 20 minutes in the steam 

 sterilizer, on 3 successive days. 



Milk. Separated milk (readily obtained from cream- 

 eries) should be used, as otherwise the cream is trouble- 

 some. If the milk is fresh it may be at once sterilized, 

 but if acid, first neutralize with sodium carbonate solu- 

 tion. Sterilization is best effected by steaming for 20 

 minutes on 3 successive days. 



Litmus Milk is made by adding sufficient litmus 

 solution to neutralized milk, to give a distinct blue 

 tinge. The milk is then sterilized in the usual manner. 



Potatoes. Select fair sized smooth potatoes and 

 wash thoroughly. Slice off the ends, and with a cork 

 borer, slightly smaller than the test tubes to be used, 

 cut out cylinders from 1^ to 2 inches long. Cut these 

 cylinders diagonally across and allow them to soak in a 

 .5% solution of sodium carbonate for one hour. Now 

 place in the bottom of the test tubes to be used, a small 

 plug of moistened absorbent cotton and then put in each, 

 one of the potato plugs. Sterilize in the autoclave for 

 45 minutes at 120°C. 



Preparation of Utensils. 

 Test Tubes. If the tubes are new, rinse several 

 times in water and then swab out with strong nitric 

 acid. Thoroughly rinse through water and allow to 

 dry on a draining rack. When dry, rinse out with 

 methylated spirit and again permit to dry. 



Plug these tubes with raw cotton and sterilize in the 

 dry air oven, at 150°C for 1 hour. 



