How Bacteria are Studied 37 



many species to get into separate tubes pure 

 cultures of each species by itself. This is 

 accomplished by what is called the "plate 

 culture," the details of which are as follows: 

 Suppose we have a mixture, say a sample of 

 impure drinking water, which contains four 

 different species of bacteria, which we wish 

 to get into pure cultures in separate tubes. 



We mix a small amount of the bacteria- 

 containing water with a much larger amount of 

 the above-described nutrient gelatin, melted 

 by heat. Then we pour this mixture into a 

 shallow glass dish with a glass cover, called a 

 Petri dish, which has been carefully sterilized 

 by heat, so as to form a thin layer. This 

 soon cools and becomes solid. The Petri 

 dish is covered to keep out any bacteria which 

 may be floating in the air, and to prevent its 

 drying, and is set away at the temperature 

 of the living-room. The individual bacteria 

 which were scattered through the gelatin layer 

 will presently commence to grow. 



After a few hours or days, as the case may 

 be, if we look at the gelatin-film (see Plate III) 

 we see, sometimes with the naked eye, some- 



