BACTERIA 



229 



through several inches (twelve or fifteen) of clean sand. Is 

 a sand filter effective ? (6) Make cultures of milk. How 

 does fresh milk compare in the number of its bacteria with 

 that which has stood for some time ? (7) What is meant by 

 Pasteurizing milk? If possible, visit a milk station where 

 milk for babies is sold. What measures render it safe ? 



FIG. 107. Inoculating gelatin tubes with platinum loop 

 Note the way iu which cotton plugs are held between the fingers 



Additional experiments. (1) Scrape the surface of a silver 

 'oin with a sterile knife and make a plate culture. Compare 

 \vith cultures made from copper coins and paper money. No 

 paper money is used in the Hawaiian Islands because of the 

 danger of transmitting disease. (2) Make plate cultures from 

 the surface of a pencil that a child has used for some time ; 

 from the edge of a common drinking cup, door handle, straps 



