EXERCISE 43 



HOW DO BACTERIA ACT ON MILK, AND HOW MAY MILK BE PRESERVED FROM 



SUCH ACTION? 



Materials. Test tubes ; cotton ; skim milk ; funnel ; rubber and glass tubing ; 

 pinch clamp ; wire basket ; formalin ; steam sterilizer (may be made with common 

 materials as directed below) ; gummed labels. 



Directions. Pupils should work in groups of two. Each group should be pro- 

 vided with six clean, dry test tubes and enough cotton to make plugs for the tubes. 

 Place about 1 inch of skim milk in each test tube. Avoid getting milk on the 

 mouth or upper inner part of the tube by using a large funnel fitted with a rubber 

 connecter with a pinch clamp ; the end of the rubber tube holds a glass tube drawn 

 out to a small opening. Pour the milk into the funnel. The proper volume of milk 

 may be allowed to enter the tube by controlling the pinch clamp. Immediately 

 plug the test tubes with cotton. Set one tube in the rack and with a gummed 

 label mark it " 1." 



Place the other five tubes in a wire basket and treat as described below. Place 

 the tubes in a steam sterilizer. Sterilizers such as are often used for sterilizing milk 

 for small children will be as satisfactory as the laboratory type. Keep the water in 



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