38 THE STORY OF THE BACTERIA. 



of colony indicates a difference in the species 

 of bacteria composing it, but nothing is 

 simpler than, directly under the microscope, to 

 take out on the tip of a sterilized platinum 

 wire little bits from each one of the different 

 forms of colonies, and transfer them to separate 

 tubes of gelatin. Thus we secure "pure cult- 

 ures " of all the different forms of bacteria 

 which were contained in the original mixture. 

 Thus, minute as the individual bacteria are, 

 lying far below the power of unaided vision, 

 we are able to manipulate them with as much 

 certainty as the agriculturist does his larger 

 plants. 



When we have thus got different species of 

 bacteria separated from one another in the 

 form of pure cultures we can experiment on 

 them in many ways, and learn their varying 

 characteristics. We can plant them under 

 such conditions that their oxygen supply is 

 limited, and learn whether they do or do not 

 thrive ; we can see whether they grow best at 

 high or low temperatures, and what degrees 

 of heat or cold will kill them ; we can grow 

 them in large quantities, and study the chemi- 



