THE STORY OF THE BACTERIA. 35 



It is obviously of the greatest importance, 

 as we have seen above, that we should be able 

 to separate different species of bacteria from 

 one another in the living condition, so that we 

 may have growths or colonies which shall con- 

 tain one species alone without admixture with 

 any other. These are called " pure cultures." 

 This is by no means an easy task, as will be 

 appreciated when we consider how exceedingly 

 minute the organisms are, and how much dan- 

 ger there is that the bacteria floating every- 

 where invisibly in the air, may become mixed 

 with those forms which we are studying. By 

 a very simple device elaborated by Dr. Koch, 

 of Berlin, we are, nevertheless, able at any 

 time to separate one species from another with 

 the utmost certainty, or from a mixture of 

 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. 



